HAWASSA UNIVERSITY
INSTITUTE OF TECHNOLOGY
FACULTY OF CIVIL ENGINEERING AND BUILT
ENVIRONMENT
DEPARTMENT OF CIVIL ENGINEERING
PERFORMANCE EVALUATION OF ROUNDABOUTS:
(A CASE STUDY IN HAWASSA CITY)
Prepared by: Name
ID. No
1. Daniel Addis …………………………………….0381/06
2. Hayder Abdella .....................................................0018/07
3. Mintesnot Tibebu...................................................0998/06
A Thesis, Submitted to School of Civil Engineering in
Partial Fulfillment for BSc. Degree in Civil Engineering
DECEMBER 2020
HAWASSA, ETHIOPIA
HAWASSA UNIVERSITY
INSTITUTE OF TECHNOLOGY
DEPARTMENT OF CIVIL ENGINEERING AND BUILT
ENVIRONMENT
SCHOOL OF CIVIL ENGINEERING
A Thesis, Submitted to School of Civil Engineering in
Partial Fulfillment for BSc. Degree in Civil Engineering
PERFORMANCE EVALUATION OF ROUNDABOUTS:
(A CASE STUDY IN HAWASSA CITY)
Prepared by: Daniel Addis
(Tech/0381/06)
Hayder Abdella
(Tech/0018/07)
Mintesnot Tibebu
(Tech/0998/06)
December 2020
HAWASSA, ETHIOPIA
DECLARATION
We hereby declare that this thesis (entitled PERFORMANCE EVALUATION OF
ROUNDABOUTS: A Case Study in Hawassa City) has been carried out by the group members
under the supervision and continuous advice of Instructor Belette (MSc), School of Civil
Engineering, Institute of Technology, Hawassa University, during the academic year of 2019/20.
We further declare that this work is our original work and has not been presented and submitted
to any other University or Institution for the award of any degree or diploma.
Name
Daniel Addis
Signature
Email Address
Phone No
danbefkadu@gmail.com
+251923162967
Hayder Abdella
hayderabdella2487@gmail.com
+251926183263
Mintesnot Tibebu
mintesnot.tibebu18@gmail.com
+251919195497
CONFIRMATION
This thesis has been submitted for examination with my approval as a University advisor.
Advisor Name
Signature
Email Address
Belette Tsegaye
belettetsegaye@gmail.com
EXAMINERS
Examiner 1
Examiner 2
Signature:
Signature:
Examiner 3
Examiner 4
Signature:
Signature:
i
ACKNOWLEDGEMENT
We would like to thank our almighty God for everything that he did for us. We wonder to give
great acknowledgement to our advisor Instructor Belette Tsegaye (MSc.) for his guidance and
encouragement to get us familiar with civil engineering concepts and impeccable support from the
beginning of the project to the final form and also his constructive criticism was fundamental in
the accomplishment of the project.
We would also like to thank Hawassa University School of Civil Engineering for introducing such
a useful research program that helped us to acquire great knowledge and understanding about our
field of study. We are so grateful to those who helped us by giving some instruments to be used on
site surveying. Specials thanks to our families, for their financial support, encouragement and
prayers, their continued support throughout our University education. Last but not the least; we
would like to acknowledge owners and administrative staff of Shalom College for helping us use
their building for video recording purpose.
ii
Table of Contents
DECLARATION ............................................................................................................................ i
ACKNOWLEDGEMENT............................................................................................................. ii
List of Tables .................................................................................................................................. v
ACRONYMS ................................................................................................................................ vii
ABSTRACT ................................................................................................................................. viii
1
INTRODUCTION .................................................................................................................. 1
1.1
Background .................................................................................................................................... 1
1.2
Statement of the Problem ............................................................................................................... 2
1.3
Significance of the Research .......................................................................................................... 2
1.4
Objectives of the Research ............................................................................................................. 2
1.4.1
General Objective................................................................................................................... 2
1.4.2
Specific Objectives................................................................................................................. 2
1.4.3
Research Questions ................................................................................................................ 3
Scope and Limitations of the Study ............................................................................................... 3
1.5
2
3
4
1.5.1
Scope of the Study ................................................................................................................. 3
1.5.2
Limitations of the Study ......................................................................................................... 3
LITERATURE REVIEW ...................................................................................................... 4
2.1
Introduction .................................................................................................................................... 4
2.2
Roundabouts................................................................................................................................... 4
2.2.1
Geometric Elements of Roundabouts ..................................................................................... 7
2.2.2
Characteristics of Roundabouts.............................................................................................. 9
2.2.3
Types of Roundabouts..........................................................................................................10
2.2.4
Capacity of Roundabouts .....................................................................................................16
MATERIALS AND METHODS ......................................................................................... 26
3.1
General .........................................................................................................................................26
3.2
Study Area....................................................................................................................................26
3.3
Materials.......................................................................................................................................27
3.4
Methods........................................................................................................................................27
3.4.1
Method of Data Collection ...................................................................................................27
3.4.2
Methods of Analysis ............................................................................................................40
ANALYSIS AND DISCUSSION ......................................................................................... 47
Manual Analysis ..........................................................................................................................47
4.1
4.1.1
Movement of Traffic ............................................................................................................47
4.1.2
Capacity Evaluation and Determination of Degree of Saturation ........................................52
iii
Delay Calculation and Determination of Level of Service ..................................................54
4.1.3
Software Analysis ........................................................................................................................56
4.2
4.2.1
Movement Summary ............................................................................................................57
4.2.2
Capacity ...............................................................................................................................58
4.2.3
Degree of Saturation ............................................................................................................59
4.2.4
Delay ....................................................................................................................................60
4.2.5
Level of Service Summary ...................................................................................................61
Comparison on Manual Analysis and Software Analysis ............................................................62
4.3
5
4.3.1
Traffic Movement ................................................................................................................63
4.3.2
Capacity Evaluation .............................................................................................................64
4.3.3
Degree of Saturation ............................................................................................................64
4.3.4
Delay ....................................................................................................................................65
4.3.5
Level of Service ...................................................................................................................65
CONCLUSION AND RECOMMENDATION .................................................................. 67
5.1
CONCLUSION ............................................................................................................................67
5.2
RECOMMENDATION ...............................................................................................................68
REFERENCES............................................................................................................................. 69
APPENDICES .............................................................................................................................. 73
Appendix A ..............................................................................................................................................73
Appendix B ..............................................................................................................................................75
iv
LIST OF TABLES
Table 2.1. Set of arrangements of lanes at the entries and exits of Turbo-roundabouts (Brilon W.
2014) .............................................................................................................................................. 14
Table 2.2. Roundabout category comparison (FHWA-SA-10-006, 2000) .................................... 16
Table 2.3. HCM Critical Gap and Follow-up Times (HCM 2010, Exhibit 17-37) ....................... 23
Table 3.1. Summary of Goduguada Roundabout Geometry on Each Legs ................................... 29
Table 3.2. Raw vehicles flow data on Mekuriyaye approach leg of Goduguada Roundabout on
November 14, 2020........................................................................................................................ 31
Table 3.3. Raw vehicles flow data on Arabsefer approach leg of Goduguada Roundabout on
November 14, 2020........................................................................................................................ 32
Table 3.4. Raw vehicles flow data on Southstar approach leg of Goduguada Roundabout on
November 14, 2020........................................................................................................................ 33
Table 3.5. Raw vehicles flow data on Campus approach leg Goduguada Roundabout on
November 14, 2020........................................................................................................................ 34
Table 3.6. Conversion factors for passenger car equivalents (pcu) (UKDOT, 1993) ................... 35
Table 3.7. Raw total traffic flow on Mekuriyaye approach leg of Goduguada Roundabout ......... 36
Table 3.8. Raw total traffic flow on Arabsefer approach leg of Goduguada Roundabout ............ 37
Table 3.9. Raw total traffic flow on Southstar approach leg of Goduguada Roundabout ............. 38
Table 3.10. Raw total traffic flow on Campus approach leg of Goduguada Roundabout ............. 39
Table 3.11. Level-of-Service Criteria (NCHRP 2010) .................................................................. 44
Table 4.1. Traffic volume count in terms of PCU ......................................................................... 48
Table 4.2. Summary of 60-minute traffic volume and Movement type for Goduguada
Roundabout .................................................................................................................................... 51
Table 4.3. Summarized performance analysis results on the roundabout ..................................... 57
Table 4.4. Summarized movement performance results on each lane........................................... 58
Table 4.5. Total capacity per movement vs Demand flow of Goduguada Roundabout ................ 59
Table 4.6. Traffic Volume from the software and count ............................................................... 64
Table 4.7. Capacity Evaluation Result........................................................................................... 64
Table 4.8. Degree of Saturation result from the Manual and Software analysis ........................... 65
Table 4.9. Delay results from software and manual analysis ........................................................ 65
Table 4.10. Level of Service results from different manual analyses and software analysis ........ 66
v
LIST OF FIGURES
Figure 2.1. Basic Geometric Features of a Modern Roundabout (FHWA-RD-00-067, 2000) ....... 5
Figure 2.2. Comparisons of vehicle-vehicle conflicts points for intersection with four single-lane
approaches (FHWA-RD-00-067, 2000) .......................................................................................... 6
Figure 2.3. Key roundabout characteristics (FHWA-SA-10-006, 2000) ....................................... 10
Figure 2.4. Roundabout design features (FHWA-SA-10-006, 2000) ............................................ 10
Figure 2.5. Mini-Roundabout with a design according to guidelines (Brilon W. 2014) ............... 11
Figure 2.6. Critical design for a Mini-Roundabout: (Brilon W. 2014) .......................................... 12
Figure 2.7. Examples of compact single-lane roundabouts (Brilon W. 2014) .............................. 13
Figure 2.8. Examples for semi-two-lane roundabouts in peripheral urban situations (Brilon W.
2014) .............................................................................................................................................. 13
Figure 2.9. Examples of Turbo-roundabouts (Brilon W. 2014) .................................................... 15
Figure 2.10. Analytical versus empirical methods (Mallikarjuna P, 2014) ................................... 18
Figure 2.11. Capacity comparison of single-lane and double-lane roundabouts (FHWA-SA-10006, 2000) ...................................................................................................................................... 25
Figure 3.1. Location of Goduguada Roundabout (GoogleET Maxar Technologies) .................... 26
Figure 3.2. Analysis of one-legged roundabout (HCM 2000) ....................................................... 41
Figure 3.3. Flow Stream Definitions (HCM 2000) ........................................................................ 41
Figure 3.4. General layout of Goduguada roundabout (SIDRA Intersection 8.0 Software) ......... 46
Figure 4.1. Degree of Saturation of Goduguada Roundabout ....................................................... 60
Figure 4.2. Control delay of Goduguada Roundabout ................................................................... 61
Figure 4.3. Level of Service Summary of Goduguada Roundabout .............................................. 62
vi
ACRONYMS
AACRA
Addis Ababa City Road Authority
AASHTO
American Association of State Highway and Transportation Officials
CETUR
Centre d’Etudes des Transports Urbains
CSA
Central Statistical Agency
ERA
Ethiopian Roads Authority
FHWA
Federal Highway Administration
HCM
Highway Capacity Manual
HCS
Highway Capacity Software
HV
Heavy Vehicles
LOS
Level of Service
LV
Light Vehicles
NCHRP
National Cooperative Highway Research Program
PCE
Passenger Car Equivalents
PCU
Passenger Car Units
SETRA
Service d’Etudes Techniques des Routes et Autoroutes
SIDRA
Signalized Intersection Design and Research Aid
UK DOT
United Kingdom Department of Transport
US DOT
United States Department of Transportation
vii
ABSTRACT
Performance Evaluation of Roundabouts has a great deal of contribution on the reduction of traffic
accidents, improvement on the performance of the roundabouts and the accessibility of
roundabouts to all users. In Hawassa city there are many problems regarding roundabouts; some
of the significant ones are congested traffic flow, insufficient lane width, delay of vehicles, long
queues, unbalanced number of entry lanes and circulatory lanes, and inadequacy of inscribed circle
diameter. The significance of this research mainly is to check whether or not study has been
conducted to objectively determine efficiency of this alternative as compared to a serious of
coordinated signalized intersections. The objective of the research is to assess the performance of
the roundabouts, to study the movement of traffic at Goduguada roundabout, to perform delay and
level of service analysis on the roundabout, and to perform delay and level of service analysis on
the roundabout, and to perform software analysis using SIDRA INTERSECTION version 8.0
software and compare results with manual calculations. The main source of information for this
research was obtained through data collection, site survey/ measurement, and recorded video to
attain comprehensive understanding of problems that occur on the study area. Depending on the
objectives of the research random sampling method by the systematic way was used. The collected
data was then analyzed and interpreted in the results and discussion section. The results from the
analysis show that the overall performance of the roundabout is excellent and this may lead to a
conclusion that roundabouts are appropriate in such types of junctions. This has its own positive
impact on the economic development of the city. Although many alternate routes are constructed
over the years due to the newly established Industry Park there is a need to consider to make
improvements on other roundabouts of Hawassa city accordingly to Goduguada roundabout.
viii
1
1.1
INTRODUCTION
Background
The history of roundabouts is almost as long as that of signalized intersections. The first roundabout
concept was introduced in 1877 by French architect Eugene Henard (De-Argao 1992). In 1903 he
suggested that the roundabout is a convenient form of traffic control when many roads converged.
The first engineering-based design guide was published by the United Kingdom Ministry of
Transport in 1929 and design formulas were introduced in 1957. The concept of modern
roundabouts was introduced in 1963 when the British government employed the off-side rule based
on which the priority was given to the circulating vehicles on the roundabout. The introduction of
flare and deflection concepts further assisted roundabouts to prevail as one of the most popular,
safe, and convenient traffic-control options in Europe and Australia (De-Argao 1992).
Evaluation of roundabout capacity is very important since it is directly related to delay, level of
service, accident, operation cost, and environmental issues. For more than three decades modern
roundabouts have been used successfully throughout the world as a junction control device. The
effect of the roundabout is that traffic is required to slow down to negotiate the curve around the
central island, but unlike full stop and signal controlled intersections, vehicles entering a
roundabout are not required to stop completely. This makes the facility more efficient under a wide
range of traffic volumes, as motorists only need to find an acceptable gap for entrance (Estifanos
M, 2019).
Ethiopia also has its share of roundabouts. Some of them currently are being changed to signalized
intersections without conducting detailed investigation. Only little attention has been paid to the
design and performance evaluation of the roundabouts, there is only little known about their
capacities and level of services (Estifanos M, 2019).
Hawassa city though not as developed as Addis Ababa is, has about seven roundabouts around St.
Gebriel Church, Welde Amanuel Square, Wanza, Fikir Hayk, Goduguada, Central Hotel, Referral
Hospital and South Star International Hotel. These roundabouts are introduced for the purpose of
traffic control purposes and are used in place of signalized intersections. The research plans to
carry out detailed investigation on the roundabout situated around Goduguada. Roundabouts before
design and construction need detailed investigation with respect to delay, capacity, and level of
service.
1
1.2
Statement of the Problem
According to Estifanos M. (2019) roundabouts are increasingly recognized as an intersection
control strategy that can fulfill multiple performance goals related to sustainability, liability,
complete streets, context-sensitive design, economic development, and others. Some transportation
agencies have recently constructed or approved the use of a serious of roundabouts on an urban
highway rather than the traditional solution of signalized intersections. While anecdotal reports
suggest that functionality interdependent roundabouts on a corridor are successful in meeting
performance goals, little research has been conducted to objectively determine the efficiency of
this alternative as compared to a serious of coordinated signalized intersections.
1.3
Significance of the Research
If the objective of this study is to be achieved the following advantages can be attained;

Reducing the congestion on the roundabout specially during peak hours;

Increasing the efficiency of road network;

Increasing safety;

Improving traffic flow and traffic operations.
1.4
Objectives of the Research
1.4.1 General Objective
The aim of this thesis in general is to make performance evaluation of roundabouts at Hawassa city
specifically Goduguada Roundabout.
1.4.2 Specific Objectives

To study the movement of traffic at the selected roundabout; the number of vehicles turn
left, right, go through, turning and total number of vehicles.

To analyze the capacity of the Goduguada roundabout and determine its degree of
saturation.

To perform delay and level of service analysis on the roundabout.

To perform software analysis using SIDRA INTERSECTION Version 8.0 software and
compare results with manual calculations.
2
1.4.3 Research Questions
In line with the discussions tried to make above, the research aims at answering the following
questions:

What is the condition of movement of traffic on Goduguada roundabout?

How is the capacity and degree of saturation of the roundabout analyzed?

What is the magnitude of delay on the respective legs of the roundabout?

What is the level of service of respective legs of the roundabout?

What differences are likely to occur in the software analyses and the manual calculations?
1.5
Scope and Limitations of the Study
1.5.1 Scope of the Study
The research was mainly conducted in Hawassa city and Evaluation of performance of roundabout
on Goduguada Roundabout.
1.5.2 Limitations of the Study
Some of the major difficulties during the research are discussed below.

Covid 19 Pandemic Disease

Recording video at the appropriate position due to lack of willingness of building owners
of the area.

The study areas considered at the beginning of the research are now under maintenance.
Hence, the scope/ study area of the research was changed to Goduguada roundabout. The
selected alternate roundabouts pre Covid 19 were:


Tesfaye Gizaw Building

Southstar Hotel

Gebriel Church

Fikr Hayk
Measurements on the geometry of the roundabout were not properly taken due to:

Shortage of time,

Revised rules and regulations of the University,

Traffic flow during day time.
3
2
2.1
LITERATURE REVIEW
Introduction
Brilon (2005) estimated capacity of roundabouts and clarified the traffic condition where a
roundabout function. Troutbeck (1994) estimated delays at intersections and showed the condition
for adopting roundabouts. Some safety researches focused on the number of accidents before and
after installing roundabouts have also been conducted. Flannery et al. (1996) showed that the
number of accidents at roundabouts reformed from unsignalized intersections decreased.
The Florida Roundabout Design Guide compared one-lane or two-lane roundabouts to signalized
intersections with one-lane or two-lane approaches and one exclusive left turn lane. It concluded
that the performance of signalized intersections is superior under heavy entering volume, while the
roundabout works better under light entering volume in terms of delay. Akcelik (1997) reported
that this study failed to consider flare effects correctly as it took under consideration only the
number of lanes and not the lane width. Old versions of the U.S. Highway Capacity Manual (U.S.
HCM) and Highway Capacity Software (HCS) are limited in their ability to provide detailed
analysis of roundabouts. HCM 2000 version tries to bridge this gap by introducing the procedures
for the study of roundabouts.
2.2
Roundabouts
According to ERA Geometric Design Manual (2013) Roundabouts are intersections of two or more
roads that are made up of a one way-circulating roadway that has priority over approaching traffic.
Priority is given to traffic already on the roundabout. Roundabouts thus operate by deflecting the
vehicle paths to slow the traffic and promote yielding. They also have a good safety record largely
because traffic speeds are low and the number of potential traffic conflicts is greatly reduced (ERA
Geometric Design Manual 2013). A roundabout is a channelized intersection at which all traffic
moves anticlockwise around a central traffic island (AACRA Geometric Design Manual, 2003).
Yield signs control the approaching traffic and the driver can only make a right turn onto the
circulating roadway. The only decision the entering drivers need to make once they reach the yield
line is whether or not a gap in the circulating traffic is large enough for them to enter. The vehicles
then exit the circulating roadway by making a right turn toward their destination (NCHRP Report
672 2010). Roundabouts are often confused with traffic circles or rotaries and it is important to be
able to differentiate between the two. According to NCHRP-2010 information guide, roundabouts
have five main characteristics that identify them when compared to traffic circles. These
4
characteristics are traffic control, priority to circulating vehicles, pedestrian access, parking and
direction of circulation.
Besides to those five mentioned above, Thaweesak (1998) included additional features of
roundabout, approach flare and deflection, which distinguish them from other traffic circles.
According to the capacity study of roundabouts in the UK, geometric elements of roundabouts play
an important role in the efficiency of roundabouts operational performance. Good geometric design
will improve safety, which is a major concern for road design in addition to capacity. Basic
parameters for design consideration of roundabouts are: design vehicles, design speed, site
distance, deflection, central island circulating width, inscribed circle diameter, entry and exit angle,
splitter island, super elevation and drainage, pavement markings, signage, lighting and landscaping
(Thaweesak, 1998).
Figure 2.1. Basic Geometric Features of a Modern Roundabout (FHWA-RD-00-067, 2000)
Figure 2.1 provides a review of the basic geometric features and dimensions of a roundabout. The
elements are described in section 2.2.1.
According to Federal Highway Administration of the United States (FHWA 2000) basic reasons
for the increased safety level at roundabouts are:

Roundabouts have fewer conflict points in comparison to conventional intersections. The
potential for conflicts, such as right angle and left turn head-on crashes is eliminated with
roundabout use. Single-lane approach roundabouts produce greater safety benefits than
5
multilane approaches because of fewer potential conflicts between road users, and
pedestrian crossing distances are short.

By installing a modern roundabout in place of other conventional intersection traffic control
types, conflict points are reduced from 32 to 8, a 75% reduction in conflict points (see
Figure 2.2).

Low absolute speeds associated with roundabouts allow drivers more time to react to
potential conflicts, also helping to improve the safety performance of roundabouts.

Since most road users travel at similar speeds through roundabouts, i.e., roundabouts have
low relative speeds, crash severity can be reduced compared to some traditionally
controlled intersections.
The figure below shows that a roundabout eliminates vehicle-vehicle crossing conflicts by
converting all movements to right turns. Separate turn lanes and traffic control (traffic signs or
signalization) can often reduce but not eliminate the number of crossing conflicts in space and/or
time. However, the most severe crashes at signalized intersections occur when there is a violation
of the traffic control device designed to separate conflicts by time (e.g., a right-angle collisions due
to a motorist running a red light, or vehicle pedestrian collisions). The ability of roundabouts to
reduce conflicts through physical, geometric features has been demonstrated to be more effective
than the reliance on driver obedience to traffic control devices. At intersections with more than
four legs, a roundabout or pair of roundabouts may sometimes be the most practical alternative to
minimize the number of conflicts.
Figure 2.2. Comparisons of vehicle-vehicle conflicts points for intersection with four single-lane
approaches (FHWA-RD-00-067, 2000)
6
Major reasons for the consideration of roundabouts on intersections are:

Unique geometric flexibility

Fit almost anywhere

Flexible - easy to modify

Provide better turning radii for trucks

Require very small sight distances

Long life if designed properly
Roundabouts may be appropriate for the following conditions:

T intersections with stop signs; high delay

Higher left and right-turning movements

More than four legs

Intersections with high crash rates

High speed four-way intersections

Future growth resulting in changeable patterns

Traffic calming purposes
Roundabouts may not be appropriate for the following conditions

Highly unbalanced flows

Design cannot handle large/oversize vehicles

Isolated roundabout in a system of coordinated signals

Traffic flows leaving roundabout interrupted by downstream traffic control
After completion of design and construction of roundabouts the following constraints may be
present

There are several ways to get it wrong regarding flexibility

Design process complex/iterative

Can be expensive at high volumes

High volume roundabouts are difficult for Motorcycles & pedestrians movements
2.2.1 Geometric Elements of Roundabouts
The geometric elements of roundabouts are defined as follows in the FHWA guide (FHWA, 2000):

Inscribed diameter: the diameter measured between the outer edges of the roadway. This
includes the circulating roadway, truck apron and the central island.

Entry curve: entry curve is used to deflect and slow entering vehicles to an appropriate
7
speed to safely circulate in the roundabout.

Exit curve: exit curve is generally bigger/flatter than the entry curve to allow vehicles to
exit at faster speed to improve traffic capacity and flow.

Entry width: the width of the entry where it meets the inscribed circle, measured
perpendicularly from the right edge of the entry to the intersection point of the left edge
line and the inscribed circle.

Apron: if required on smaller roundabouts to accommodate the wheel tracking of large
vehicles, an apron is the mountable portion of the central island adjacent to the circulatory
roadway.

Exit width: the width of the exit where it meets the inscribed circle, measured
perpendicularly from the right edge of the exit to the intersection point of the left edge line
and the inscribed circle.

Holding line: holding line is pavement marking that defines where the vehicles have to
give-way to the circulating traffic. It is generally marked along the inscribed circle.

Central island: it is the raised area in the center of a roundabout around which traffic
circulates.

Circulating carriageway: circulating carriageway is a curved path used by vehicles to travel
around the central island. It is defined by line marking.

Circulating carriageway width: circulating carriageway width defines the roadway width
for vehicle circulation around central island. It has to be wide enough to accommodate the
largest design vehicles turning path.

Circulating path radius: the minimum radius on the fastest through path around the central
island.

Circulating volume: circulating volume is the total volume in a given period of time on the
circulatory roadway immediately prior to an entrance.

Circulating speed: the speed vehicles travel at while on the circulatory roadway

Entry angle: term used in the United Kingdom regression models. It serves as a geometric
proxy for the conflict angle between entering and circulating streams and is determined
through a geometric construct.

Entry path curvature: term used in the United Kingdom to describe a measure of the amount
of entry deflection to the right imposed on vehicles at the entry to a roundabout.

Entry path radius: the minimum radius on the fastest through path prior to the yield line.
8

Exit path radius: the minimum radius on the fastest through path into the exit.

Entry speed: the speed a vehicle is traveling at as it crosses the yield line.

Entering traffic: vehicles located on a roundabout entrance.

Exiting traffic: vehicles departing a roundabout by a particular exit.

Entering volume: the total volume in a given period of time on an entrance to a roundabout.

Splitter island: a raised or painted area on an approach used to separate entering from
exiting traffic, deflect and slow entering traffic, and provide storage space for pedestrians
crossing that intersection approach in two stages. Also known as a median island or a
separator island.
2.2.2 Characteristics of Roundabouts
Modern roundabouts have superior operational characteristics (i.e. capacity, delay, queue length,
proportion stopped, etc.). The capability of reducing the frequency of crashes and crash severity
makes it safer than other traffic control devices (Russell, et al., 2000). Circular intersection forms
have been part of the transportation system in the United States for over a century. Their
widespread usage decreased after the mid-1950s, as rotary intersections began experiencing
problems with congestion and safety. However, the advantages of the modern roundabout,
including modified and improved design features, have now been recognized and put to the test in
the United States. There are now estimated to be well over a thousand roundabouts in the United
States and tens of thousands worldwide, with the number estimated to be increasing in the United
States each year (FHWA-SA-10-006). A modern roundabout has the following distinguishing
characteristics and design features which are illustrated in fig 2.3 and fig 2.4

Channelized approaches

Yield control on all entries

Counterclockwise circulation of all vehicles around the central island
9
Figure 2.3. Key roundabout characteristics (FHWA-SA-10-006, 2000)
Figure 2.4. Roundabout design features (FHWA-SA-10-006, 2000)
In traffic circles, weaving is unavoidable and weaving sections have to accommodate conflicting
movements whereas in a roundabout vehicle are sorted by destination at the approach. Therefore,
weaving at the roadway is minimized.
2.2.3 Types of Roundabouts
According to Brilon W. (2014) modern roundabouts have become a common element in urban
traffic networks. The design standards as they have been developed over the times since 1985 have
been proven to be successful regarding aspects of design and traffic performance. There are about
six types of roundabouts which will be discussed below.
10

Compact single-lane roundabouts with diameters between 26m and 40m

Mini-roundabouts with a traversable island and diameters between 13m 1nd 22m

Larger roundabouts (40m-60m) with 2-lane access for cars and single-lane operation for
trucks (semi-two-lane)

Turbo-roundabouts

Signalized roundabouts
2.2.3.1 Mini-Roundabouts
Mini-roundabouts can only be applied in urban areas with a general speed limit of 50 km/h. They
have an inscribed diameter between 13m and 24m. Compact vehicles like cars or vans have to
follow the circular lane whereas larger vehicles like big trucks or busses are allowed to override
the central island as far as necessitated by their kinematics. It is important that the central island is
elevated by curbs with at least 40mm to 60mm. It should also be paved by a material different from
the circular lane; e.g. concrete block pavement. Otherwise the rule not to override the central
reserve is not well accepted. On figure 2.5 the central island built of concrete is on the same level
as the circular lane (lack of a delta of 4cm-6cm). This reduces the acceptance of a correct driving
on the circular lane. The direct access from the circle to private parking positions (left side) gives
reason for several conflicts.
Figure 2.6 illustrates one more recent experience is that separate cycle tracks crossing the entries
and exits adjacent the pedestrian crosswalks fail to be a safe solution. That is, any markings of
cycle facilities should be avoided at mini roundabouts. Cyclists can most safely be accommodated
by driving on the circular lane. Mini-roundabouts where the center island is just represented by
some road marking have failed to achieve sufficient safety.
Of course, mini-roundabouts have only single-lane entries and exits without any flaring. The
circular lane should have a width between 4m and 6m and it should be inclined to the outside by
around 2.5% relative to the approaching streets.
Figure 2.5. Mini-Roundabout with a design according to guidelines (Brilon W. 2014)
11
Figure 2.6. Critical design for a Mini-Roundabout: (Brilon W. 2014)
Another surprising experience is that mini-roundabouts are often favored especially by truck
operators and transit bus companies. They prefer mini-roundabouts since ease of handling for the
large vehicles is advantageous in comparison to normal roundabouts.
One primarily unexpected aspect was that at mini-roundabouts special attention must be paid to
sight distances. At each entry there must be good visibility especially into the next upstream entry
(on to the left side). If this visibility is obstructed by buildings or plantations then priority accidents
must be suspected.
2.2.3.2 Compact Single-Lane Roundabouts
The single-lane roundabout is the reference type to which all other types are compared. This basic
type of a roundabout is applied on rural roads and for urban streets as well. However,
recommendations for design are slightly different. This standard type has an inscribed diameter
between 26m and 45m in an urban environment and 30m to 50m at a rural intersection. This
characteristic feature is that all entries and exits as well as the circulating carriageway have only
one single lane. The two figures on figure 2.7 try to illustrate the above explanation about single
lane roundabouts.
12
Figure 2.7. Examples of compact single-lane roundabouts (Brilon W. 2014)
2.2.3.3 Compact Two-Lane Roundabouts
There is an intermediate type of roundaout which could be called loosely translated compact twolane roundabout. Roundabouts of this type have a diameter between 40m and 60m and a circualr
carriageway between 8m and 10m wide without lane marking on the circle. Beside these measures
they are very much designed like a single-lane roundabout and they would even be treated like
single-lane.
Figure 2.8. Examples for semi-two-lane roundabouts in peripheral urban situations (Brilon W.
2014)
The figures above show two-lane roundabouts that are often treated as single lane roundabouts
specially in the United States. The figure on the left resmbles like a single lane roundabout although
it is a two-lane roundabout, since it does not have any lane marks on it. The one to the right has a
dotted mark just between the two vehicles though it is not a lane mark.
13
These types could have two lane entries if required. But two-lane exits are strictly forbidden due
to safety reasons. On these roundabouts passenger cars could drive side by side. Most drivers,
however, prefer to drive in a staggered manner. Larger vehicles are forced to use the whole width
of the circular carriageway. This may cause some path overlap between vehicles entering on the
same approach. This is however not a problem in practice.
This type of roundabout becomes a problem in connection with pedestrians or cyclists. Cyclits on
the roundabout must be strictly prohibited. It is recommended that for these non-motorized road
users facilities separated from the roundabout should be provided. Especially at the cross-points
within two-lane entries, crosswalks of any kind are strictly discouraged due to safety reasons. Thus,
this type can mainly be used in rural envronments and in peripheral urban areas where no
pedestrians or cyclists must be expected. Despite of these limitations many roundabouts of this
type have been built at many places and they are operated quite successfully.
2.2.3.4 Turbo-Roundabouts
Turbo-roundabout is a term which goes back to engineers in Netherlands, especially Bertus
Fortuijn. Independently from that source Turbos had also been developed in Germany. A turboroundabout is a roundabout with alternating numbers on the circle. The characteristic properties
are, that the addition of a lane is only achieved by adjoining the lane on the inner side opposite
from an entry. Subtracting of a lane can be either performed as a normal lane drop or by a two-lane
exit where the right lane must leave into the exit. Thus, the typical arrangements are shown on the
table below.
Table 2.1. Set of arrangements of lanes at the entries and exits of Turbo-roundabouts (Brilon W.
2014)
Type
1
2
3
4
Entry
Exit
14
Figure 2.9. Examples of Turbo-roundabouts (Brilon W. 2014)
In Netherlands, at turbo-roundabouts, the lanes on the entries and exits are diverted by vertical
elements which can not be overrun by cars. Due to considerations of motorcycle safety and winter
service, but also costs and maintenance requirements, the vertical elements in the middle of the
intersection are not accepted. They are also not required.
2.2.3.5 Signalized Roundabouts
What many people would not expect is that a combination of traffic signals arranged at a circular
intersection can be an excellent solution for larger traffic volumes. There are some examples which
have proven that this combination can be capable to carry up to 50000veh/day or even
60000veh/day on a two-lane circualr carriageway with a diameter beyond 50m.
Many people think that concepts of roundabouts and signals contradict. But this opinion is
absolutely wrong. There are however some confusing ways of signalization which are required to
acieve high capacity. These ideas have first been formulated in the UK(Crabtree, 1992). An
overview about the methodology and a report on German experience has been given in(Brilon W.
1995). With the application of these methods a rather high capacity on a limited area of land can
be achieved. In the city of Hannover several such examples are operated with success. But in
specific cases the potential may also be limited depending on the pattern of movemnets at the
intersction.
Accoring to FHWA (2006) roundabouts have been classified into three basic categories according
to size and number of lanes to facilitate discussion of specific performance or design issues: miniroundabouts, single-lane roundabouts, and multilane roundabouts. These are summarized in the
table below.
15
Table 2.2. Roundabout category comparison (FHWA-SA-10-006, 2000)
Design Element
Mini Roundabout
Single-Lane
Multi-Lane
Roundabout
Roundabout
Desirable maximum
15mph to 20mph
20mph to 25mph
25mph to 30mph
entry design speed
(25km/h to 30km/h)
(30km/h to 40km/h)
(40km/h to 50km/h)
Maximum number of
1
1
2+
Typical inscribed
45ft to 90ft (13m to
90ft to 180ft (27m to
150ft to 300ft (46m
circle diameter
27m)
55m)
to 91m)
Central island
Fully traversable
Raised (may have
Raised (may have
traversable apron)
traversable apron)
entering lanes per
approach
treatment
Typical daily service
Up to approximately
Up to approximately
Up to approximately
volumes on 4-leg
15000veh/day
25000veh/day
45000veh/day for
roundabout below
two-lane roundabout
which may be
expected to operate
without requiring a
detailed capacity
analysis(veh/day)
2.2.4 Capacity of Roundabouts
According to Federal Highway Agency of United States the maximum flow rate that can be
accommodated at a roundabout entry depends on two factors: the circulating flow on the
roundabout that conflicts with the entry flow, and the geometric elements of the roundabout. When
the circulating flow is low, drivers at the entry are able to enter the roundabout without significant
delay. The larger gaps in the circulating flow are more useful to the entering drivers and more than
one vehicle may enter each gap. As the circulating flow increases, the size of the gaps in the
circulating flow decrease, and the rate at which vehicles can enter also decreases. Note that when
computing the capacity of a particular leg, the actual circulating flow to use may be less than
16
demand flows, if the entry capacity of one leg contributing to the circulating flow is less than
demand on that leg (FHWA-RD-00-067).
The geometric elements of the roundabout also affect the rate of entry flow. The most important
geometric element is the width of the entry and circulatory roadways, or the number of lanes at the
entry and on the roundabout. Two entry lanes permit nearly twice the rate of entry flow as does
one lane. Wider circulatory roadways allow vehicles to travel alongside, or follow, each other in
tighter bunches and so provide longer gaps between bunches of vehicles. The flare length also
affects the capacity. The inscribed circle diameter and the entry angle have minor effects on
capacity.
As at other forms of un signalized intersection, when traffic flows on an approach exceed
approximately 85 percent of capacity, delays and queue lengths vary significantly about their mean
values (with standard deviations of similar magnitude as the means). For this reason, the analysis
procedures in some countries (Australia, Germany, and the United Kingdom), recommend that
roundabouts be designed to operate at no more than 85 percent of their estimated capacity.
As performance data become available for roundabouts designed according to the procedures used
in the United States, they will provide a basis for development of operational performance
procedures specifically calibrated for U.S. conditions. Therefore, analysts should consult future
editions of the Highway Capacity Manual.
2.2.4.1 Methods of Roundabout Capacity Analysis
The Highway Capacity Manual (HCM, 2010) defines the capacity of a facility as ‘the maximum
hourly rate at which persons or vehicles can reasonably be expected to traverse a point or uniform
section of a lane or roadway during a given time period under prevailing roadway, traffic and
control conditions. Capacity is the main determinant of the performance measures such as delay,
queue length and stop rate. The relationship between a given performance measure and capacity is
often expressed in terms of degree of saturation (demand volume - capacity ratio). Capacity is the
maximum sustainable flow rate that can be achieved during a specific time period under prevailing
road, traffic and control conditions. Capacity represents the service rate (queue clearance rate) in
the performance (delay, queue length, stop rate) functions, and therefore is relevant to both under
saturated and over saturated conditions (Akcelik, 2011).
There are two distinct theories or methodologies to assess the capacity of the roundabouts. These
theories are:

Empirical method

Analytical or gap acceptance method
17
Analytical vs. Empirical Methods
Analytical Method
Geometry, traffic
volumes
Gap acceptance
behavior
Empirical Method
Capacity
Figure 2.10. Analytical versus empirical methods (Mallikarjuna P, 2014)
The figure above illustrate the basic differences between the analytical and empirical methods. The
analytical method uses the gap acceptance model while the empirical method uses only the
geometry and the traffic volume of the roundabout. Further discussion on the two methods will be
made below.
2.2.4.1.1 Empirical Method
This model correlates geometric features and performance measures, such as capacity, average
delay and queue length, through regression of field data. In this way, a relationship (generally linear
or exponential) between the entering flow of an approach and the circulating flow in front of it
(Rodergerdts et al. 2004) is generated. This model is better than analytical ones but requires an
oversaturated condition for calibration and may have poor transferability to different countries of
the world.
2.2.4.1.2 Analytical Method
This model can be developed from uncongested sites; the driver on the approach (entering flow)
needs to select an acceptable gap in the circulating stream, to carry out the entering maneuver. The
gap is the headway between two consecutive vehicles on the circulating flow; so, the “critical gap”
(tc) is the minimum headway accepted by a driver in the entering stream. If the gap accepted is
larger than minimum, then more than one driver can enter the roundabout; the headway between
two consecutive vehicles in the entering flow, which utilizes the same gap, is defined as “followup time” (tf). So, the analytical model analyses the roundabout capacity as a function of the critical
gap, the follow-up time and the circulating flow. However, for capacity evaluation, the following
are some assumptions due to the nature of geometry, turning movement, vehicle types and approach
grade:
18

Constant values for “tc” and “tf”

Exponential distribution for the gaps into the circulating flow

Constant traffic volumes for each traffic flow
These specific assumptions make the use of these models difficult in practice. Furthermore, there
are limitations, such as:

The estimation of the critical gap is not easy;

The geometric factors are not directly taken into account;

The inconsistent gaps are not accounted for in theory (forced right of way when traffic is
congested, circulating drivers give up right of way, different gap accepted by different
vehicles, the rejection of large gap before accepting a smaller one, etc).
2.2.4.1.3 Empirical Method versus Analytical Method
Kimber in his initial laboratory report (1980) states that the dependence of entry capacity on
circulating flow depends on the roundabout geometry. Kimber defined five geometric parameters
which have an effect on the capacity as mentioned earlier (Section 2.2). These are entry width and
flare, the inscribed circle diameter (a line that bisects the center island and the circulating lane
twice), and the angle and radius of the entry.
Similarly, Kimber in his paper (1989) states that gap acceptance is not a good estimator of capacity
in the United Kingdom. He further states that, single-lane entries are the basis for the simplest case
for gap acceptance models; while, empirical models apply also to multilane entries. Kimber reasons
that gap acceptance models do not increase capacity correctly when additional entry lanes are
added. Perhaps Kimber’s reasoning in this publication was due to its creation date. Many new ideas
have been put forth on how additional lanes affect capacity in a gap acceptance model.
Kimber makes two interesting comments in his paper (1989), the first being that many
circumstances exist where driver response to yield signs conforms to gap acceptance assumptions.
However, he questions whether or not gap acceptance is a sufficient description of this interaction.
The main flaw of the gap acceptance theory is that it poorly evaluates capacity for at-capacity
roundabouts. Flannery et al. (1998) comment that congested roundabouts are very scarce in the
United States. Therefore, the empirical regression model might be difficult to use since it requires
a saturated facility to be calibrated.
The second comment by Kimber is that because of driver behavior and geometric variation, is not
safe to transfer theories from one country to another. Fisk, in a 1991 article, agreed that regression
models should not be transferred from region to region, or between roundabouts of different
geometrical configurations. Fisk argues that because a regression model requires a great deal of
19
data for calibration, it may work well at a specific facility, but cannot be universal. Furthermore,
Fisk feels that gap acceptance models demonstrated reliable predictions for both capacity and delay
of New Zealand roundabouts. Fisk believed that by changing vehicle class parameters or providing
a range of critical gap values, gap acceptance modeling could be used in other locations.
Akcelik (ARR 321, 1998) contends that while Kimber objects to the “simple gap acceptance
method”, the model presented for use in the SIDRA software package goes beyond the simple
approach. One main addition to Akcelik’s gap acceptance approach is the modeling of the
roundabout based on approach lane use. Furthermore, Akcelik writes that the method presented in
his report improves capacity prediction during heavy flow conditions and especially for multilane
roundabouts with uneven approach demands.
Many of the additional elements used in SIDRA are parameters used to enhance its basic gap
acceptance theory. The parameters that deal with the entering traffic stream include the inscribed
diameter, average entry lane width, the number of circulating and entry lanes, the entry capacity
(based on the circulating flow rate), and the ratio of the entry flow to the circulating flow.
These additional model elements demonstrate the detailed nature of the SIDRA model (AACRA
also recommend SIDRA for capacity evaluation). Another important component of Akcelik’s
formulation is the identification of the dominant and sub-dominant entry lanes based on their flows.
The dominant lane has the highest flow rate, and all others are subdominants. The purpose of this
component is that dominant and sub-dominant entry lanes can have different critical gap and follow
up times. The distinction between dominant and sub-dominant lanes appears to be quite important
because vehicles using the leftmost entry lane must find a gap in both circulating lanes, as opposed
to the right entry lane, which must only deal with traffic in the outer most circulating lane.
SIDRA also includes a passenger car equivalent (pce) factor for heavy vehicles. In this regard,
Akcelik (1997) recommended that pce per hour be used in place of vehicles per hour when the
proportion of heavy vehicles surpassed 5percent. Many other authors concurred Akcelik’s
recommendation. In their Roundabout Design Workshop (2001), Rodegertds explained that heavy
vehicles primarily affect roundabout capacity due to their size, not because of their slower
acceleration and speed. The U.S. DOT’s Roundabout Guide (2000) suggests typical PCE
conversion factors for adjusting entering and circulating volumes. These include a 1.5 factor for
recreational vehicles and buses, and a 2.0 factor for track trailers. Because the empirical
formulation has some drawbacks, for example, data has to be collected at over-saturated flow (or
at capacity) level. It is a painstaking task to collect enough data to ensure reliability of results, and
this method is sometimes inflexible under unfamiliar circumstances, for example, when the value
20
is far out of the range of regressed data. Consequently, researchers looked for other reliable
methods of determining roundabout capacity. Many researchers agree that a gap acceptance theory
(Analytical Method) is a more appropriate tool. An advantage of this method is that the gap
acceptance technique offers a logical basis for the evaluation of capacity. Secondly, it is easy to
appreciate the meaning of the parameters used and to make adjustments for unusual conditions.
Moreover, gap acceptance conceptually relates traffic interactions at roundabouts with the
availability of gap in the traffic streams (Thaweesak, 1998).
Further investigation into which theory is more appropriate shows that the gap acceptance model
is felt to be more transferable from country to country and location to location than is the empirical
regression model. List et al. (2007) investigated multilane roundabouts in New York State using
gap acceptance-based models. They commented that it is possible to transfer capacity equations
from overseas.
2.2.4.2 Capacity Models Developed Abroad
2.2.4.2.1 United Kingdom
Kimber (1980) conducted studies in the United Kingdom (UK) and developed an empirical linear
regression equation based on large number of observations at roundabouts operating at-capacity.
This equation directly relates capacity to roundabout geometry.
𝑄𝑒 = 𝑘(𝐹 − 𝑓𝑐𝑄𝑐), where Qe is entry capacity (vph)
Qc is circulating flow (pce/h)
k, F, fc are constants derived from the geometry of the roundabout
2.2.4.2.2 Australia
Troutbeck (1993) conducted studies for the Australian Road Research Board and developed an
analytical equation based on gap acceptance characteristics observed and measured at roundabouts
operating below capacity. Critical gap and follow-up times are related to roundabout geometry and
capacity is then determined using the following equation.
𝑄𝑒 =
α𝑄𝑐𝑒 −λ(𝑡𝑐−𝑡𝑚)
1−𝑒 −𝜆𝑡𝑓
, where Qe is entry capacity (vph)
Qc is circulating flow (vph)
α is proportion of non-bunched (free) vehicles in the circulating streams
λ is model parameter
tc is critical gap (s)
tf is minimum headway in circulating streams (s)
tm is follow-up time (s)
21
2.2.4.2.3 Germany
Stuwe (1992) studied eleven German roundabouts and developed the following exponential
regression equation to estimate roundabout entry capacity.
𝐵𝑉𝑐
𝐶 = 𝐴𝑒 −(10000) , where C is entry capacity, vph;
Vc is circulating flow, vph; and
A, B are parameters dependent on the number of circulating and entry lanes
2.2.4.2.4 Switzerland
Simon (1991) developed the following linear model based on studies of roundabouts in
Switzerland.
8
𝐶 = 1500 − ( ) 𝑉𝑔, where C is entry capacity (vph)
9
Vg is impeding flow (vph), determined with the equation below
𝑉𝑔 = β𝑉𝑐 + α𝑉𝑠, where Vc is circulating flow (vph)
Vs is exiting flow (vph)
α, β are parameters depending on geometry and number of circulating lanes
2.2.4.2.5 France
In 1988 the French government organization Centre d’Etudes des Transports Urbains (CETUR),
now known as CERTU, developed the following linear model for urban roundabout capacity.
5
𝐶 = 1500 − (6) 𝑉𝑔, where C is entry capacity (vph)
Vg is impeding flow (vph), determined with the equation below
𝑉𝑔 = 𝑉𝑐 + α𝑉𝑠, where Vc is circulating flow (vph)
Vs is exiting flow (vph)
α is parameter dependent on splitter island width.
Similarly, SETRA, the French national design organization for rural highways, developed a linear
equation for the capacity of rural roundabouts in 1987 as follows:
𝐶 = (1330 − 0.7𝑉𝑔)(1 + 0.1[𝑙𝑒 − 3.5]),
where C is entry capacity (vph)
Vg is impeding flow (vph), dependent on circulating and exiting flows and geometry
le is entry width (m)
2.2.4.2.6 United States
The Highway Capacity Manual 2010 (HCM) presents a methodology for estimating roundabout
capacity based on gap acceptance. The capacity model is applicable only to single-lane roundabouts
if the circulating volume is less than 1,200 vph. Insufficient experience in the U.S. precludes the
22
HCM from containing guidelines for multiple-lane roundabouts. The HCM assumes that the gap
acceptance characteristics of drivers entering a roundabout to be similar to those of drivers making
right turns a following model was developed in the U.S. the equation to be introduced below is
used in the methodology section to analyze capacity of the roundabout.
−𝑉𝑐𝑡𝑐
𝐶𝑎 =
𝑉𝑐𝑒 3600
−𝑉𝑐𝑡𝑓
, where ca = approach capacity (veh/h)
1−𝑒 3600
vc = conflicting circulating traffic (veh/h)
tc = critical gap (s)
tf = follow-up time (s)
e = constant
The HCM 2010 recommends ranges for critical gap and follow-up time, which are presented in the
table below.
Table 2.3. HCM Critical Gap and Follow-up Times (HCM 2010, Exhibit 17-37)
Bound
Critical gap (s)
Follow up time (s)
Upper bound
4.1
2.6
Lower bound
4.6
3.1
The U.S. Federal Highway Administration (FHWA) publication, Roundabouts: an Informational
Guide (2000) presents a more comprehensive discussion of roundabout performance analysis than
the HCM (2010). This document differentiates roundabouts based upon size and environment and
provides capacity models for urban compact roundabouts, typical single-lane roundabouts, and
typical double-lane roundabouts.
Urban compact roundabouts have nearly perpendicular single-lane approach legs and inscribed
circle diameters in the range of 25 to 30 m (82 to 98.5 ft.). The capacity model for urban compact
roundabouts is based on the capacity curves developed by Brilon, Wu, and Bondzio (1997) for
German roundabouts with single-lane entries and a single-lane circulatory roadway as follows:
𝑄𝑒 = 1218 − 0.74𝑄𝑐, where: Qe is entry capacity (vph)
Qc is circulating flow (vph)
The UK equation developed by Kimber (1980) form the basis for the capacity models derived for
typical single-lane and double-lane roundabouts. The indicated assumptions of geometric
parameters were chosen so as to simplify the equations as follows:
Single-lane roundabouts:
23
𝑄 = 𝑀𝑖𝑛{(1212 − 0.5447𝑄𝑐), (1800 − 𝑄𝑐)
Assuming: D = 40 m, r = 20 m, φ = 30°, v = 4 m, e = 4 m, l’ = 40 m
Double-lane roundabouts:
𝑄𝑒 = 2424 − 0.7159𝑄𝑐
Assuming: D = 55 m, r = 20 m, φ = 30°, v = 8 m, e = 8 m, l’ = 40 m.
Where, Qe is entry capacity (vph)
Qc is circulating flow (vph)
D is inscribed circle diameter (m)
r is entry radius (m)
φ is entry angle (degrees)
v is approaching half width (m)
e is entry width (m)
l' is effective flare length (m)
When capacity requirements are met at double-lane roundabouts with the gradual widening of the
approaches (flaring), the capacity of each entry lane, qmax, is estimated with the equation below.
This equation is based on Wu’s (1997) studies on the effect of short lanes on entry capacity for
two-lane roundabouts.
2𝑞
𝑞𝑚𝑎𝑥 = 𝑥 𝑛+1
√2
=
𝑞𝑚𝑎𝑥2
𝑛+1√2
,
where: q is flow in each lane (pce/h) (assumed to be equal in both lanes);
x is degree of saturation;
n is length of queue space (vehicles)
24
qmax 2 is capacity of entry at a double-lane roundabout (pce/h)
Figure 2.11. Capacity comparison of single-lane and double-lane roundabouts (FHWA-SA-10006, 2000)
The figure above shows a comparison of the expected capacity for both the single-lane and doublelane roundabouts. Again, it is evident that the number of lanes, or the size of the entry and
circulating roadways, has a significant effect on the entry capacity.
25
3
3.1
MATERIALS AND METHODS
General
To conduct the research, capacity manuals from abroad and SIDRA intersection software to
analyze quality measures such as delay, capacity, degree of saturation, and level of service of
Goduguada roundabout were used.
3.2
Study Area
Hawassa is the administrative capital for regional government of Southern Nations, Nationalities
and People’s State of Ethiopia on the shores of Lake Hawassa which is one of the Great Rift Valley
lakes. The city has an area of 47.66km2 comprising more than 225,700 people. This means 4735
people live within one square kilo meters (CSA, 2013). Astronomically the city is at 7°03'43'' North
+Latitude and 38°28'34'' East Longitude. The city is located 270 km south of Addis Ababa via
DebreZeit-Mojo, 130 km east of Sodo, 75 km north of Dilla and 1125 km north of Nairobi. The
city lies on the Trans African highways 4 Cairo-Cape Town, with a latitude and longitude of 703′N
38028′E coordinates and an elevation of 1708 meters. Hawassa is the second diverse city in terms
of population ethnicity next to Addis Ababa (Ermias K, 2015). The specific study area where the
research was conducted is Goduguada roundabout. The Google earth image below illustrates
Goduguada roundabout.
Figure 3.1. Location of Goduguada Roundabout (GoogleET Maxar Technologies)
26
3.3
Materials
Various materials were used for collecting the data used in the research. Primary data and
secondary data were collected based on the respective requirements of both the software analysis
and manual calculations.
The software analysis was carried out using SIDRA 8.0 intersection software as mentioned earlier.
This software needs both the volume data recorded from the roundabout and the geometric data
taken from Google earth pro. To perform the analysis the traffic volume data was first converted
to an hour volume data using the procedures mentioned on section 3.3.3.1. After the data was
converted to an hour volume data using the peak hour factor, it was used on the volume section of
the software. Before using the volume data the following procedures were carried out:

Layout arrangement of the roundabout accordingly to the Google map of the area in the
intersection section of the software,

Movements (origin-destination) were defined in the movement definitions section of the
software,

Dimensions (width of approach lanes), number of lanes, and lane configuration data was
filled on the lane geometry section of the roundabout,

Movement classes (light vehicles and heavy vehicles) were defined on the lane movement
section of the software,
3.4

Other geometrical information was filled in the roundabout section of the software,

Finally these information were used to analyze the performance of the roundabout.
Methods
3.4.1 Method of Data Collection
Roundabout's, geometric and traffic data (peak hour) were required in order to achieve the
objectives of this research. As much as possible, the traffic data collected should indicate the
existing peak hour traffic conditions. The peak hour traffic conditions are used to determine the
peak hour factor which is used in the capacity calculations. Therefore, it was necessary to collect
these data using the collecting materials and techniques mentioned earlier.
To achieve the aim of the study and to answer the formulated research questions, different data are
required, and these data are categorized into primary data and secondary data. Traffic volume count
data is among the primary data and it was collected using a video camera. A stopwatch was used
to identify for how long the video was recorded. As a result, after preparing measurement materials
27
and being familiar with the measurement techniques, the traffic movement data on the roundabout
were recorded, the general information on appropriate forms for traffic volume data were filled and
dimensions of geometric elements of the roundabout in section 3.4.1.1 were taken from Google
earth pro.
3.4.1.1 Geometric Data
As per the requirement of SIDRA INTERSECTION Software Version 8.0, the geometric data
collected should include all the necessary parameters for the purpose of analysis. All the necessary
geometric data of the parameters used in the software on the roundabout were taken from Google
Earth Pro images and some of the dimensions were confirmed by making measurements on the
parameters such as entry lane width, central island diameter, number of circulating lanes, and
number of entry lanes. These data were measured with a tap meter. Table 3.1. Shows the
summarized collected geometric data of Goduguada roundabout.
28
Table 3.1. Summary of Goduguada Roundabout Geometry on Each Legs
Goduguada Roundabout
Roundabout Name
Entry
No of circulatory
Central Island
Inscribed circle
Average lane
Circulating lane
lane
lane
diameter (m)
diameter (m)
width (m)
width (m)
Leg1 (Mekuriyaye)
1
1
14
30
3.5
8
Leg 2 (Arabsefer)
2
1
14
30
3.5
8
Leg 3 (Southstar)
1
1
14
30
3.5
8
Leg 4 (Campus)
2
1
14
30
3.5
8
Approach leg
29
3.4.1.2 Traffic Data
Traffic movements of vehicle and vehicles' volume classification are important parameters for
performance analysis using SIDRA 8.0 intersection design software and the manual analysis using
Highway Capacity Manual. Because of this, vehicle volume data were collected at peak hours with
their direction of movements. Peak hour traffic volume represents the most critical period for
operations and has the highest capacity requirements. The peak hour volume, however, is not a
constant value from day to day or from season to season. If the highest hourly volumes for a given
location were listed in descending order, a large variation in the data would be observed, depending
on the type of route and facility under study. Whether the design hour was measured, established
from the analysis of peaking patterns, or based on modeled demand, the peak-hour factor (PHF) is
applied to determine design hour flow rates. Peak-hour factors in urban areas generally range
between 0.80 and 0.95. Lower values signify greater variability of flow within the subject hour,
and higher values signify little flow variation. Peak hour factors over 0.95 are often indicative of
high traffic volumes, sometimes with capacity constraints on flow during the peak hour. Peak rates
of flow are related to hourly volumes through the use of the peak-hour factor. This factor is defined
as the ratio of total hourly volume to the peak rate of flow within the hour:
PHF =
Hourly Volume
Peak flow rate (within the hour)
Since 15-min periods are used, the PHF may be computed as;
V
PHF = 4∗𝑉
15
=
V
, where, PHF = peak-hour factor,
Maximum flow rate
V= hourly volume (vph), and
V15 = volume during the peak 15 min of the peak hour (veh/15 min).
Note that, even if only 15-minute interval video were used for three hours at different times of the
day data as traffic movement data, it is necessary to record traffic count data for at least two hours
at a time. The number of counted vehicles from the recorded video in each leg are shown on table
3.2 to table 3.5.
30
Table 3.2. Raw vehicles flow data on Mekuriyaye approach leg of Goduguada Roundabout on November 14, 2020
Mekuriyaye
Movement
Mekuriyaye-Arabsefer
Mekuriyaye-Southstar
Vehicle
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Type
(veh)
(veh)
(veh)
(veh)
(veh)
(veh)
Motorcycles Car Single unit truck Motorcycles Car Single unit truck Motorcycles Car Single unit truck
Time Interval
Time Interval
or bus
Time Interval
Mekuriyaye-Campus
or bus
Morning (1:30 am to 2:30am)
18
42
1:30-1:45
2
12
1
1:45-2:00
7
11
1
17
2:00-2:15
6
14
1
18
2:15-2:30
6
18
0
6:15-6:30
1
6:30-6:45
or bus
1
6
10
0
38
2
3
9
0
49
1
2
8
0
1
2
7
0
9
26
46
Afternoon (6:15am to 7:15am)
0
12
30
1
3
4
0
5
10
0
15
35
1
2
2
1
6:45-7:00
4
12
0
14
41
0
1
4
1
7:00-7:15
3
12
1
1
1
3
0
11:30-11:45
1
7
22
32
Evening (11:30 am to 12:30pm)
1
14
36
0
5
6
1
11:45:12:00
3
8
1
16
36
1
2
5
0
12:00-12:15
2
10
1
17
44
1
1
5
0
12:15-12:30
1
9
0
24
38
0
1
6
1
31
Table 3.3. Raw vehicles flow data on Arabsefer approach leg of Goduguada Roundabout on November 14, 2020
Arabsefer
Movement
Arabsefer-Southstar
Arabsefer-Campus
Vehicle
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Type
(veh)
(veh)
(veh)
(veh)
(veh)
(veh)
Motorcycles Car Single unit truck Motorcycles Car Single unit truck Motorcycles Car Single unit truck
Time Interval
Time Interval
or bus
Time Interval
Arabsefer-Mekuriyaye
or bus
Morning (1:30 am to 2:30am)
7
10
1:30-1:45
2
17
1
1:45-2:00
10
14
1
12
2:00-2:15
15
19
0
11
2:15-2:30
7
10
1
6:15-6:30
1
15
1
6:30-6:45
6
10
0
6
6:45-7:00
13
12
1
4
7:00-7:15
5
7
0
11:30-11:45
1
11:45:12:00
or bus
0
7
6
0
19
0
9
14
1
10
0
11
18
0
0
10
13
0
0
6
3
0
6
0
7
9
0
7
0
9
15
0
0
8
11
1
16
9
6
Evening (11:30 am to 12:30pm)
1
5
7
0
6
4
0
7
12
1
10
11
0
8
11
1
12:00-12:15
14
15
0
8
8
1
8
16
0
12:15-12:30
6
8
1
7
10
1
9
12
1
9
13
Afternoon (6:15am to 7:15am)
4
6
32
Table 3.4. Raw vehicles flow data on Southstar approach leg of Goduguada Roundabout on November 14, 2020
Southstar
Movement
Southstar-Campus
Southstar-Mekuriyaye
Vehicle
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Type
(veh)
(veh)
(veh)
(veh)
(veh)
(veh)
Motorcycles Car Single unit truck Motorcycles Car Single unit truck Motorcycles Car Single unit truck
Time Interval
Time Interval
or bus
Time Interval
Southstar-Arabsefer
or bus
Morning (1:30 am to 2:30am)
13
46
1:30-1:45
4
5
0
1:45-2:00
2
4
1
9
2:00-2:15
5
11
0
24
2:15-2:30
6
11
0
6:15-6:30
2
3
0
6:30-6:45
1
3
0
7
6:45-7:00
4
9
0
15
7:00-7:15
3
7
1
11:30-11:45
3
11:45:12:00
or bus
1
2
7
1
47
5
7
12
0
47
0
11
9
0
2
2
4
2
2
1
4
2
39
4
3
5
0
32
1
5
6
1
0
1
3
2
4
17
44
Evening (11:30 am to 12:30pm)
1
10
42
1
1
5
1
1
2
0
8
44
2
5
8
0
12:00-12:15
3
10
0
21
39
0
7
7
0
12:15-12:30
4
9
1
21
47
1
1
1
1
24
53
Afternoon (6:15am to 7:15am)
5
38
33
Table 3.5. Raw vehicles flow data on Campus approach leg Goduguada Roundabout on November 14, 2020
Campus
Movement
Campus-Mekuriyaye
Campus-Arabsefer
Vehicle
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Light Vehicles
Heavy Vehicles
Type
(veh)
(veh)
(veh)
(veh)
(veh)
(veh)
Motorcycles Car Single unit truck Motorcycles Car Single unit truck Motorcycles Car Single unit truck
Time Interval
Time Interval
or bus
Time Interval
Campus-Southstar
or bus
Morning (1:30 am to 2:30am)
13
8
1:30-1:45
4
2
0
1:45-2:00
6
8
1
17
2:00-2:15
3
11
0
10
2:15-2:30
8
8
0
6:15-6:30
2
2
0
6:30-6:45
4
6
0
11
6:45-7:00
1
5
0
4
7:00-7:15
5
7
0
11:30-11:45
2
11:45:12:00
or bus
0
5
7
0
13
0
7
8
1
8
2
6
7
0
1
14
12
0
0
3
5
0
9
1
5
1
1
3
1
5
6
0
0
9
2
0
1
1
7
Evening (11:30 am to 12:30pm)
1
4
6
0
3
2
1
2
4
0
8
7
1
2
1
1
12:00-12:15
1
4
0
9
7
0
5
4
0
12:15-12:30
3
5
0
6
4
1
5
1
1
2
9
Afternoon (6:15am to 7:15am)
11
7
34
The different types of vehicles in a traffic stream have different characteristics like width, length,
height and mass; these different size vehicles have different capacity impacts. Volumes are
typically expressed in passenger car vehicles per hour (v/h), to convert other vehicle types to
Passenger Car Equivalents (pce), the conversion factors are used to get passengers car equivalents
as indicated in Table 3.6.
Table 3.6. Conversion factors for passenger car equivalents (pcu) (UKDOT, 1993)
Vehicle Type
Passenger Car Equivalents (pce)
Car
1.0
Single-unit truck or bus
1.5
Truck with trailer
2.0
Bicycle or motorcycle
0.5
By using the conversion factor, the traffic volume at peak hour in the passenger car unit is
summarized in Table 3.7 to Table 3.10. The total passenger car unit volume will be used to
determine the peak hour factor in the analysis section on the next chapter.
35
Vehicle type
Vehicle type
Vehicle type
Mekuriyaye-Campus
Mekuriyaye-Southstar
Mekuriyaye-Arabsefer
Table 3.7. Raw total traffic flow on Mekuriyaye approach leg of Goduguada Roundabout
1:30-
1:45-
2:00- 2:15- 6:15- 6:30- 6:45- 7:00- 11:30- 11:45- 12:00- 12:15-
1:45
2:00
2:15
2:30
6:30
6:45
7:00
7:15
11:45
12:00
12:15
12:30
Car
12
11
14
18
9
10
12
12
7
8
10
9
Motrocycle
2
7
6
6
1
5
4
3
1
3
2
1
HV
Single unit
1
1
1
0
0
0
0
1
1
1
1
0
(veh)
truck or bus
Total PCU
15
16
19
21
10
13
14
15
9
11
13
10
%age of HV (%)
10.34
9.375
8.10
0
0
0
0
10
16.67
13.64
12
0
Car
42
38
49
46
30
35
41
32
36
36
44
38
Motrocycle
18
17
18
26
12
15
14
22
14
16
17
24
HV
Single unit
1
2
1
1
1
1
0
1
0
1
1
0
(veh)
truck or bus
Total PCU
53
50
60
61
38
44
48
45
43
46
54
50
%age of HV (%)
2.857
6.061
2.52
2.47
4
3.41
0
3.37
0
3.297
2.778
0
Car
10
9
8
7
10
9
8
7
4
2
4
3
Motorcycle
6
3
2
2
6
3
2
2
3
2
1
1
HV
Single unit
0
0
0
0
0
0
0
0
0
1
1
0
(veh)
truck or bus
Total PCU
13
11
9
8
13
11
9
8
6
5
6
4
%age of HV (%)
0
0
0
0
0
0
0
0
0
33.33
25
0
LV (veh)
LV (veh)
LV (veh)
36
Vehicle type
Vehicle type
Vehicle type
Arabsefer-Mekuriyaye
Arabsefer-Campus
Arabsefer-Southstar
Table 3.8. Raw total traffic flow on Arabsefer approach leg of Goduguada Roundabout
1:30-
1:45-
2:00- 2:15- 6:15- 6:30- 6:45- 7:00- 11:30- 11:45- 12:00- 12:15-
1:45
2:00
2:15
2:30
6:30
6:45
7:00
7:15
11:45
12:00
12:15
12:30
Car
17
14
19
10
15
10
12
7
16
12
15
8
Motrocycle
12
10
15
7
1
6
13
5
1
7
14
6
Single unit
1
1
0
1
1
0
1
0
1
1
0
1
Total PCU
25
21
27
15
17
13
20
10
18
17
22
13
%age of HV
6.12
7.31
0
10
8.82
0
7.5
0
8.333
8.824
0
12
Car
10
19
10
13
6
6
7
6
7
11
8
10
Motrocycle
7
12
11
9
4
6
4
9
5
10
8
7
HV
Single unit
0
0
0
0
0
0
0
0
0
0
1
1
(veh)
truck or bus
Total PCU
14
25
16
18
8
9
9
11
10
16
14
15
%age of HV
0
0
0
0
0
0
0
0
0
0
11.11
0
Car
6
14
18
13
3
9
15
11
4
11
16
12
Motorcycle
7
9
11
10
6
7
9
8
6
8
8
9
HV
Single unit
0
1
0
0
0
0
0
1
0
1
0
1
(veh)
truck or bus
Total PCU
10
20
24
18
6
13
20
17
7
17
20
18
%age of HV
0
7.5
0
0
0
0
0
9.09
0
9.091
0
8.333
LV (veh)
HV (veh)
truck or bus
LV (veh)
LV (veh)
37
Vehicle type
Vehicle type
Vehicle type
Southstar-Arabsefer
Southstar-Mekuriyaye
Southstar-Campus
Table 3.9. Raw total traffic flow on Southstar approach leg of Goduguada Roundabout
1:30-
1:45-
2:00- 2:15- 6:15- 6:30- 6:45- 7:00- 11:30- 11:45- 12:00- 12:15-
1:45
2:00
2:15
2:30
6:30
6:45
7:00
7:15
11:45
12:00
12:15
12:30
Car
5
4
11
11
3
3
9
7
4
2
10
9
Motrocycle
4
2
5
6
2
1
4
3
3
1
3
4
HV
Single unit
0
1
0
0
0
0
0
1
1
0
0
1
(veh)
truck or bus
Total PCU
7
7
14
14
4
4
11
10
7
3
12
13
%age of HV
0
23.08
0
0
0
0
0
15
21.43
0
0
12
Car
46
47
47
53
38
39
32
44
42
44
39
47
Motrocycle
13
9
24
24
5
7
15
17
10
8
21
21
HV
Single unit
1
5
0
2
2
4
1
0
1
2
0
1
(veh)
truck or bus
Total PCU
54
59
59
48
44
49
41
53
49
51
50
59
%age of HV
2.778
12.71
0
4.41
6.89
12.4
3.66
0
3.093
5.882
0
2.542
Car
7
12
9
4
4
5
6
3
5
8
7
1
Motorcycle
2
7
11
2
1
3
5
1
1
5
7
1
HV
Single unit
1
0
0
2
2
0
1
2
1
0
0
1
(veh)
truck or bus
Total PCU
10
16
15
8
8
7
10
7
7
11
11
3
%age of HV
15.79
0
0
37.5
40
0
15
46.2
21.43
0
0
50
LV (veh)
LV (veh)
LV (veh)
38
Vehicle type
Vehicle type
Vehicle type
Campus-Southstar
Campus-Arabsefer
Campus-Mekuriyaye
Table 3.10. Raw total traffic flow on Campus approach leg of Goduguada Roundabout
1:30-
1:45-
2:00- 2:15- 6:15- 6:30- 6:45- 7:00- 11:30- 11:45- 12:00- 12:15-
1:45
2:00
2:15
2:30
6:30
6:45
7:00
7:15
11:45
12:00
12:15
12:30
Car
2
8
11
8
2
6
5
7
1
4
4
5
Motrocycle
4
6
3
8
2
4
1
5
2
2
1
3
HV
Single unit
0
1
0
0
0
0
0
0
1
0
0
0
(veh)
truck or bus
Total PCU
4
13
13
12
3
8
6
10
4
5
5
7
%age of HV
0
12
0
0
0
0
0
0
42.86
0
0
0
Car
8
13
8
9
7
9
3
7
6
7
7
4
Motrocycle
13
17
10
2
11
11
4
1
4
8
9
6
HV
Single unit
0
0
2
1
0
1
1
0
0
1
0
1
(veh)
truck or bus
Total PCU
15
22
16
12
13
16
7
8
8
13
12
9
%age of HV
0
0
18.7
13.0
0
9.38
23.1
0
0
12
0
17.65
Car
7
8
7
12
5
1
6
2
2
1
4
1
Motorcycle
5
7
6
14
3
5
5
9
3
2
5
5
HV
Single unit
0
1
0
0
0
1
0
0
1
1
0
1
(veh)
truck or bus
Total PCU
10
13
10
19
7
5
9
7
5
4
7
5
%age of HV
0
11.54
0
0
0
30
0
0
30
42.86
0
30
LV (veh)
LV (veh)
LV (veh)
39
3.4.2 Methods of Analysis
3.4.2.1 Movement of Traffic
The first and main objective of this research is to study the movement of traffic at the selected
roundabout since it is mandatory for all the other objectives of the research. To study the number
of vehicles’ movement data collections were carried out using the following procedures.

The peak hour of the roundabout was chosen by making frequent visual surveys;

Then an appropriate tall building nearby the roundabout was selected;

Then videos for 1 hour with 15 minutes intervals were recorded in the morning, afternoon,
and evening;
After the videos were recorded, the left turn, right turn, through pass, and total number of vehicles
were identified. During the identification of the movement types, vehicle type determination was
carried out simultaneously since it is equally important for the research as the movement type and
volume.
3.4.2.2 Capacity Analysis and Determination of Degree of Saturation
This section presents procedures for the analysis of capacity and degree of saturation of
roundabouts. The data collection was carried out mainly by recording videos at peak hours for 1
hour with intervals of 15 minutes and this data was be converted to an hour data. These 15-minute
data recorded aids in calculation of conflicting traffic. The recorded video was captured from a
convenient position that can show all vehicles on the four legs that circulate on the roundabout at
an instant with a clear image. From the recorded video the number of existing vehicles at each leg
of the roundabout as well as the conflicting and circulating traffic in the circulatory lane of the
roundabout were counted. There are two models for the analysis of existing structures one that
relies on the field data to be collected to develop relationships between geometric design features
and performance measures such as capacity and delay which is known as the empirical approach.
The other method which is the analytical method which makes its basis on gap acceptance theory.
The choice of the latter approach depends on the available calibration data. The empirical suit at
conditions where there is congestions at roundabouts for calibration, but gap acceptance methods
can be developed from uncongested roundabouts. The figure below illustrates the vehicles that
circulate on the roundabout (Vc) as well as that enter the roundabout (Va) from the approach lanes.
40
Figure 3.2. Analysis of one-legged roundabout (HCM 2000)
Overview of the Gap Acceptance Method
There are two basic parameters that can be used to estimate the capacity of a roundabout which are
critical gap and follow-up time. These methods are further explained and compared in section
2.2.4.1. We assumed the performance of the roundabout on the four legs can be analyzed
independently and consequently. On many capacity performance evaluation analyses, information
gathered only on one of the legs are used. Basically, approach flow and circulating flows are used
in the analysis’s procedure. Figure 3.3 shows vehicle denotations and their flow direction of all the
twelve vehicles (V1 to V12) in and around the roundabout.
Figure 3.3. Flow Stream Definitions (HCM 2000)
Steps to Perform Roundabout Capacity Analysis based on Highway Capacity Manual 2000
The required procedures to carry out a roundabout analysis are listed below. This procedure is only
applicable to a single-lane roundabout with circulating flows less than 1200 veh/h.
41

Define the existing geometry and traffic conditions for the intersection under study. For
each leg, movement traffic volume data are entered for each approach.

Determine the conflicting (circulating) traffic at each leg of the roundabout. For each leg,
the approach and the circulating traffic is computed. If the circulating flow exceeds 1200
veh/h, this procedure should not be used, unless field data have been collected for the
critical gap and follow-up time,

Determine the capacity of the entry lanes using the equation below,

Assess the general performance of the roundabout based on the v/c ratio.
−𝑉𝑐𝑡𝑐
𝐶𝑎 =
𝑉𝑐𝑒 3600
−𝑉𝑐𝑡𝑓
, where ca = approach capacity (veh/h)
1−𝑒 3600
vc = conflicting circulating traffic (veh/h)
tc = critical gap (s)
tf = follow-up time (s)
e = constant
The parameters in the equation can be determined as follows

First information from the roundabout like movement volumes will be used

Peaking approach flow traffic volumes will be adjusted using the formula below
𝑉𝑥 =

𝑣𝑜𝑙𝑢𝑚𝑒
𝑃𝐻𝐹
, where, PHF= Peak hour factor of the roundabout
Circulating flows will then be determined using the following equations
Vc, ne = 𝑉9 + 𝑉11 + 𝑉12, where, Vc, ne is volume of traffic circulating in the northeast
direction of the roundabout
Vc, nw = 𝑉2 + 𝑉3 + 𝑉12, where, Vc, nw is the volume of traffic circulating in the northwest
direction of the roundabout
Vc, sw = 𝑉3 + 𝑉5 + 𝑉6, where, Vc, sw is the volume of traffic circulating in the southwest
direction of the roundabout
Vc, se = 𝑉6 + 𝑉8 + 𝑉9, where, Vc, sw is the volume of traffic circulating in the southeast
direction of the roundabout

Upper bound and lower bound capacities of each leg will then be calculated using this
−𝑉𝑐𝑡𝑐
formula
𝐶𝑎 =
𝑉𝑐𝑒 3600
−𝑉𝑐𝑡𝑓
1−𝑒 3600

Approach volumes at each legs will then be calculated as follows:
42
Va, ne = 𝑉1 + 𝑉2 + 𝑉3, where V a, ne is the approach traffic volume in the northeast direction
Va, nw = 𝑉4 + 𝑉5 + 𝑉6, where V a, nw is the approach traffic volume in the northwest direction
Va, sw = 𝑉7 + 𝑉8 + 𝑉9, where V a, sw is the approach traffic volume in the southwest direction
Va, se = 𝑉10 + 𝑉11 + 𝑉12, where V a,
se
is the approach traffic volume in the southeast
direction

After the approach volumes in each direction are calculated volume to capacity ratio can be
determined. The ratio helps estimate the degrees of saturation in each leg of the roundabout.
𝑣
𝑐
𝑛𝑒 =
𝑉𝑎,𝑛𝑒
𝐶𝑛𝑒𝑏
, where V a, ne is the approach volume in the northeast direction
C neb is the approach capacity of the northeast bound direction
𝑣
𝑐
𝑛𝑤 =
𝑉𝑎,𝑛𝑤
𝐶𝑛𝑤𝑏
, where V a, nw is the approach volume in the northwest direction
C nwb is the approach capacity of the northwest bound direction
𝑣
𝑠𝑤 =
𝑐
𝑉𝑎,𝑠𝑤
𝐶𝑠𝑤𝑏
, where V a, sw is the approach volume in the southwest direction
C swb is the approach capacity of the southwest bound direction
𝑣
𝑠𝑒 =
𝑐
𝑉𝑎,𝑠𝑒
𝐶𝑠𝑒𝑏
, where V a, se is the approach volume in the southeast direction
C seb is the approach capacity of the southeast bound direction
3.4.2.3 Delay Calculation and Determination of Level of Service
Roundabouts tend to treat all movements at an intersection equally, with no priority provided to
major movements over minor movements. Each approach is required to yield to circulating traffic,
regardless of whether the approach is a local street or major arterial. This may result in more delay
to the major movements than might otherwise be desired. This problem is most acute at the
intersection of high-volume major streets with low-volume to medium-volume minor streets (e.g.,
major arterial streets with minor collectors or local streets). Therefore, the overall street
classification system and hierarchy should be considered before selecting a roundabout (or stopcontrolled) intersection. This limitation should be specifically considered on emergency response
routes in comparison with other intersection types and control. The delays depend on the volume
43
of turning movements and should be analyzed individually for each approach, according to the
following procedure
The first step in this procedure is calculating delay using the formula below:
3,600
( ci ) 𝑥𝑖
3,600
√(𝑥
2
di =
+ 900𝑇 [𝑥𝑖 − 1 +
] + 5 ∗ min[𝑥𝑖 , 1]
𝑖 − 1) +
𝑐𝑖
450𝑇
Where i = eastbound, northbound, westbound, and southbound approaches
di = average control delay for approach i (seconds/vehicle)
xi = volume-to-capacity ratio of approach
ci = capacity of approach i (vehicles/hour)
T = analysis time period (hour)
The parameters in the above equation can be determined as follows:

Xi is the V/C ratio is to be determined from the analyses carried out in the degree of
saturation determination section.

T is usually 15 minutes or 0.25h but in our case it is 1hour since the analysis period is 1h

Ci is capacity of approach to be determined from the analyses carried out in the capacity
computation section
After delay calculations are carried out the level of service the roundabout can be determined
using the table below. Table 3.11 shows the criteria for level of service by volume-to-capacity
ratio using control delay as the primary measure of quality.
Table 3.11. Level-of-Service Criteria (NCHRP 2010)
Control delay (s/veh)
0-10
>10-15
>15-25
>25-35
>35-50
>50
Level of Service by volume-to-capacity ratio
v/c ≤ 1.0
v/c >1.0
A
F
B
F
C
F
D
F
E
F
F
F
44
3.4.2.4 Performance Evaluation using SIDRA Intersection software
SIDRA INTERSECTION 8.0 software has used the gap-acceptance methodology for roundabout
capacity estimation where gap-acceptance parameters are estimated from the roundabout
geometry. In short, SIDRA INTERSECTION 8.0 has employed a combined (hybrid) geometry
and gap-acceptance modelling approach in order to consider the effect of roundabout geometry on
driver behavior directly through gap-acceptance modelling. This approach accepts the importance
of roundabout geometry but states that roundabout geometry alone is not good enough and
modelling of driver behavior is needed for roundabout capacity estimation, just as it is needed for
modelling the capacity of any other intersection type (signals, sign control).
There has been a lot of debate about the relative benefits of models based on "gap acceptance
theory" vs "empirical models" in the past, with claims that gap-acceptance modelling does not work
for roundabouts. Much of this debate has been misleading due to simplistic model categorization
based on the suggestion that these modeling approaches are mutually exclusive. Opinion has also
been expressed that it does not matter which one of these modelling approaches is used. This has
overlooked the fact that SIDRA INTERSECTION 8.0 has used the gap-acceptance method not
only for capacity but also for performance estimation including unique equations for back of queue
and stop rate based on modelling of gap-acceptance cycles. Although several other empirical delay
studies and procedures exist for delay analysis, SIDRA offers a unique advantage in delay studies.
While the empirical studies are limited within the study of the stopped delay (or stopped and
queuing delay), SIDRA can conveniently calculate total delay by summing geometric delay,
queuing delay, the acceleration and deceleration delay, and stopped delay. Expected output from
the software after the analysis is complete are:

Capacity of the roundabout

Degree of saturation

Delay

Level of service
The figure below is extracted from SIDRA INTERSECTION 8.0 software and it illustrates the
general layout of the Goduguada roundabout.
45
Figure 3.4. General layout of Goduguada roundabout (SIDRA Intersection 8.0 Software)
46
4
ANALYSIS AND DISCUSSION
The analysis and discussion section is the most important part of the study. This section is
principally considered based on the predefined research objectives. During the study the analysis
was done on Goduguada roundabout using the collected data and following the procedures
mentioned from the research methodology section with the aid of SIDRA Intersection Version 8.0
software, the Highway Capacity Manual, and NCHRP-Report 672. According to the methods
mentioned in the methodology section the expected results are obtained from manual calculations
and are compared with the results obtained from the software analyses. The results are presented
in the following sections with brief explanations.
4.1
Manual Analysis
4.1.1 Movement of Traffic
The movement of traffic is the most essential part of this research as it is a requirement to carry out
all analyses carried out. Without determining the traffic movement and data it is impossible to carry
out any manual calculations or software analyses. Just before the traffic data was taken, the peak
hour of the roundabout was determined by collecting traffic data on different times of day from the
1 hour video recorded from second floor of Shalom College. The recorded videos were then
interpreted in the following manner.
The videos were recorded from 1:30am-2:30am, 6:15am-7:15am, and 11:30am-12:30am in the
morning, afternoon, and evening respectively for intervals of 15 minutes in each hour. The data
obtained from the recorded video is tabulated tables 3.2 to 3.5. As mentioned earlier in section
3.3.2.2 the collected raw data which was converted into passenger car equivalents (Table 3.7 to
Table 3.10) will be used here to determine the peak hour factor of each legs of the roundabout. The
analysis will be carried out as follows.
The first step in this computation was to find the total traffic volume for each 15-minute period in
terms of passenger car units which is tabulated in Tables 3.7 to 3.10 using the conversion factors
on table 3.6. Once the total traffic volume count was determined in terms of PCU, the 15-minute
interval with the highest volume will be known for each leg.
47
Table 4.1. Traffic volume count in terms of PCU
2:00-
2:15-
6:15-
6:30-
6:45-
7:00-
11:30-
11:45-
12:00-
12:15-
1:45
2:00
2:15
2:30
6:30
6:45
7:00
7:15
11:45
12:00
12:15
12:30
Mekuryaye
Arabsefer
15
16
19
21
10
13
14
15
9
11
13
10
21
Southstar
53
50
60
61
38
44
48
45
43
46
54
50
61
Campus
13
11
9
8
13
11
9
8
6
5
6
4
13
Southstar
25
21
27
15
17
13
20
10
18
17
22
13
27
Campus
14
25
16
18
8
9
9
11
10
16
14
15
25
Mekuryaye
10
20
24
18
6
13
20
17
7
17
20
18
24
Campus
7
7
14
14
4
4
11
10
7
3
12
13
14
Mekuryaye
54
59
59
48
44
49
41
53
49
51
50
59
59
Arabsefer
10
16
15
8
8
7
10
7
7
11
11
3
16
Mekuryaye
4
13
13
12
3
8
6
10
4
5
5
7
13
Arabsefer
15
22
16
12
13
16
7
8
8
13
12
9
16
Southstar
10
13
10
19
7
5
9
7
5
4
7
5
19
Campus
Southstar
Origin
1:45-
Arabsefer
Peak
1:30-
Destination
15min
volume
48
The peak hour volume is the sum of the volumes of the four 15-minute time intervals within the
peak hour. The peak hour for all legs in this case appears to be from 1:30AM-2:30AM, since all
the peak 15min volumes are found within this hour. The actual (design) flow rate can be calculated
by dividing the peak hour volume by the PHF, or by multiplying the peak 15min volume by four
(4 ∗ 𝑉15). In other words. The actual flow rate is equal to the maximum flow rate.
Peak Hour Volume (Mekuriyaye-Arabsefer) = 15+16+19+21 =71pcu (The peak 15-minute volume
was 21pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
71
=
Maximum flow rate 4 ∗ 21
=
0.85
The actual (design) flow rate (Mekuriyaye-Arabsefer): 4*21= 84pcu/hr
Peak Hour Volume (Mekuriyaye-Southstar) = 53+50+60+61 = 224pcu (The peak 15-minute
volume was 61pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
224
=
Maximum flow rate 4 ∗ 61
=
0.92
The actual (design) flow rate (Mekuriyaye-Southstar): 4*61= 244pcu/hr
Peak Hour Volume (Mekuriyaye-Campus) = 13+11+9+8 = 41pcu (The peak 15-minute volume
was 13pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
41
=
Maximum flow rate 4 ∗ 13
=
0.79
The actual (design) flow rate (Mekuriyaye-Campus): 4*13= 52pcu/hr
Peak Hour Volume (Arabsefer-Southstar) = 25+21+27+15 = 88pcu (The peak 15-minute volume
was 27pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
88
=
Maximum flow rate 4 ∗ 27
=
0.81
The actual (design) flow rate (Arabsefer-Southstar): 4*27= 108pcu/hr
Peak Hour Volume (Arabsefer-Campus) = 14+25+16+18 = 73pcu (The peak 15-minute volume
was 25pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
73
=
Maximum flow rate 4 ∗ 25
=
0.73
49
The actual (design) flow rate (Arabsefer-Campus): 4*25= 100pcu/hr
Peak Hour Volume (Arabsefer-Mekuriyaye) = 10+20+24+18 = 72pcu (The peak 15-minute
volume was 24pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
72
=
Maximum flow rate 4 ∗ 24
=
0.75
The actual (design) flow rate (Arabsefer-Mekuriyaye): 4*24= 96pcu/hr
Peak Hour Volume (Southstar-Campus) = 7+7+14+14 = 42pcu (The peak 15-minute volume was
14pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
42
=
Maximum flow rate 4 ∗ 14
=
0.75
The actual (design) flow rate (Southstar-Campus): 4*14= 56pcu/hr
Peak Hour Volume (Southstar-Mekuriyaye) = 54+59+59+48 = 220pcu (The peak 15-minute
volume was 59pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
220
=
Maximum flow rate 4 ∗ 59
=
0.93
The actual (design) flow rate (Southstar-Mekuriyaye): 4*59= 236pcu/hr
Peak Hour Volume (Southstar-Arabsefer) = 10+16+15+8 = 49pcu (The peak 15-minute volume
was 16pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
49
=
Maximum flow rate 4 ∗ 16
=
0.77
The actual (design) flow rate (Southstar-Arabsefer): 4*16= 64pcu/hr
Peak Hour Volume (Campus-Mekuriyaye) = 4+13+13+12 = 42pcu (The peak 15-minute volume
was 13pcu in this leg)
PHF =
V
4 ∗ 𝑉15
=
V
42
=
Maximum flow rate 4 ∗ 13
=
0.81
The actual (design) flow rate (Campus-Mekuriyaye): 4*13= 52pcu/hr
Peak Hour Volume (Campus-Arabsefer) = 15+22+16+12 = 65pcu (The peak 15-minute volume
was 22pcu in this leg)
50
V
4 ∗ 𝑉15
PHF =
=
V
65
=
Maximum flow rate 4 ∗ 22
=
0.74
The actual (design) flow rate (Campus-Arabsefer): 4*22= 88pcu/hr
Peak Hour Volume (Campus-Southstar) = 10+13+10+19 = 52pcu (The peak 15-minute volume
was 19pcu in this leg)
V
4 ∗ 𝑉15
PHF =
=
V
52
=
Maximum flow rate 4 ∗ 19
=
0.68
The actual (design) flow rate (Campus-Southstar): 4*19= 76pcu/hr
Therefore, the governing peak hour factor of the roundabout will be 0.93
Table 4.2. Summary of 60-minute traffic volume and Movement type for Goduguada
Roundabout
Vehicle
V1
Movement
Mekuriyaye-
Movement
Traffic
Peak Hour
Adjusted Traffic
Type
(pcu/hr)
Factor
(pcu/hr)
Right Turn
71
0.93
77
Through
224
0.93
241
Arabsefer
V2
MekuriyayeSouthstar
V3
Mekuriyaye-Campus
Left Turn
41
0.93
45
V4
Arabsefer-Southstar
Right Turn
88
0.93
95
V5
Arabsefer-Campus
Through
73
0.93
79
V6
Arabsefer-
Left Turn
72
0.93
78
Mekuriyaye
V7
Southstar-Campus
Right Turn
42
0.93
46
V8
Southstar-
Through
220
0.93
237
Mekuriyaye
V9
Southstar-Arabsefer
Left Turn
49
0.93
53
V10
Campus-Mekuriyaye
Right Turn
42
0.93
46
V11
Campus-Arabsefer
Through
65
0.93
70
V12
Campus-Southstar
Left Turn
52
0.93
56
51
4.1.2 Capacity Evaluation and Determination of Degree of Saturation
Degree of saturation or volume to capacity ratio provides a direct assessment of the sufficiency of
a given roundabout. While there are no absolute standards for the degree of saturation, the
Australian design procedure suggests that the degree of saturation for an entry lane should be less
than 0.85 for satisfactory operation. When the degree of saturation exceeds this range, the operation
of the roundabout will likely deteriorate rapidly. Volume to capacity ratio can be computed
simultaneously with capacity using the procedures mentioned in section 3.3.3.2.
Volume adjustment
𝑉𝑥 =
𝑒𝑥𝑖𝑠𝑡𝑖𝑛𝑔 𝑣𝑜𝑙𝑢𝑚𝑒
𝑃𝐻𝐹
71
𝑉1 = 0.93 = 77𝑣𝑒ℎ/ℎ
88
𝑉4 = 0.93 = 95𝑣𝑒ℎ/ℎ
42
𝑉7 = 0.93 = 46𝑣𝑒ℎ/ℎ
42
𝑉10 = 0.93 = 46𝑣𝑒ℎ/ℎ
224
𝑉2 = 0.93 = 241𝑣𝑒ℎ/ℎ
73
𝑉5 = 0.93 = 79𝑣𝑒ℎ/ℎ
220
𝑉8 = 0.93 = 237𝑣𝑒ℎ/ℎ
65
𝑉11 = 0.93 = 70𝑣𝑒ℎ/ℎ
41
𝑉3 = 0.93 = 45𝑣𝑒ℎ/ℎ
72
𝑉6 = 0.93 = 78𝑣𝑒ℎ/ℎ
49
𝑉9 = 0.93 = 53𝑣𝑒ℎ/ℎ
52
𝑉12 = 0.93 = 56𝑣𝑒ℎ/ℎ
Circulating flow computation
Vc, ne = 𝑉9 + 𝑉11 + 𝑉12 = 53 + 70 + 56 = 179veh/h
Vc, nw = 𝑉2 + 𝑉3 + 𝑉12 = 241 + 45 + 56 = 342veh/h
Vc, sw = 𝑉3 + 𝑉5 + 𝑉6 = 45 + 79 + 78 = 202veh/h
Vc, se = 𝑉6 + 𝑉8 + 𝑉9 = 78 + 237 + 53 = 368veh/h
Approach flow computation
Va, ne = 𝑉1 + 𝑉2 + 𝑉3 = 77 + 241 + 45 = 363veh/h
Va, nw = 𝑉4 + 𝑉5 + 𝑉6 = 95 + 79 + 78 = 252veh/h
Va, sw = 𝑉7 + 𝑉8 + 𝑉9 = 46 + 237 + 53 = 336veh/h
Va, se = 𝑉10 + 𝑉11 + 𝑉12 = 46 + 70 + 56 = 172veh/h
52
Capacity computation
−𝑉𝑐𝑡𝑐
𝐶𝑎 =
𝑉𝑐𝑒 3600
−𝑉𝑐𝑡𝑓
1 − 𝑒 3600
Northeast direction
−179∗4.1
𝐶𝑎𝑢𝑏 =
179𝑒 3600
−179∗2.6
−179∗4.6
= 1204veh/h
𝐶𝑎𝑙𝑏 =
1−𝑒 3600
179𝑒 3600
−179∗3.1
= 997veh/h
1−𝑒 3600
Northwest direction
−342∗4.1
𝐶𝑎𝑢𝑏 =
342𝑒 3600
−342∗2.6
−342∗4.6
= 1059veh/h
𝐶𝑎𝑙𝑏 =
1−𝑒 3600
342𝑒 3600
−342∗3.1
= 867veh/h
1−𝑒 3600
Southwest direction
−202∗4.1
𝐶𝑎𝑢𝑏 =
202𝑒 3600
−202∗2.6
−202∗4.6
= 1183veh/h
𝐶𝑎𝑙𝑏 =
1−𝑒 3600
202𝑒 3600
−202∗3.1
= 686veh/h
1−𝑒 3600
Southeast direction
−368∗4.1
𝐶𝑎ub =
368𝑒 3600
−368∗2.6
−368∗4.6
= 1037veh/h
1−𝑒 3600
𝐶𝑎𝑙𝑏 =
368𝑒 3600
−368∗3.1
= 847veh/h
1−𝑒 3600
Vehicle-to-capacity ratio
Northeast direction
𝑣
𝑐
𝑛𝑒𝑢𝑏 =
363
= 0.3
1204
𝑣
𝑐
363
𝑛𝑒𝑙𝑏 = 997 = 0.36
Northwest direction
𝑣
𝑐
𝑛𝑤𝑢𝑏 =
252
1059
= 0.24
𝑣
𝑐
𝑛𝑤𝑙𝑏 =
252
867
= 0.29
Southwest direction
𝑣
𝑐
𝑠𝑤𝑢𝑏 =
336
1183
= 0.28
𝑣
𝑐
𝑠𝑤𝑙𝑏 =
336
686
= 0.49
Southeast direction
𝑣
𝑐
𝑠𝑒𝑢𝑏 =
172
1037
= 0.17
𝑣
𝑐
𝑠𝑒𝑙𝑏 =
172
847
= 0.2
53
While the HCM does not define a standard for volume-to-capacity ratio, international and domestic
experience suggests that volume-to-capacity ratios in the range of 0.85 to 0.90 as mentioned above
represent an approximate threshold for satisfactory operation. When the degree of saturation
exceeds this range, the operation of the roundabout enters a more unstable range in which
conditions could deteriorate rapidly, particularly over short periods of time. Delay begins to
increase exponentially. A volume-to-capacity ratio of 0.85 should not be considered an absolute
threshold; in fact, acceptable ratios may be achieved at higher ratios. Where an operational analysis
reaches the volume-to-capacity ratio of 0.85, it is encouraged to conduct additional sensitivity
analysis to evaluate whether relatively small increments of additional volume have domestic
impacts on delays or queues. It is also recommended to take a closer look at the assumptions used
in the analysis. A higher volume-to-capacity ratio during peak periods may be a better solution
than the potential physical and environmental impacts of excess capacity that is unused most of the
day.
The first leg which is the northeast bound has a volume-to-capacity ratio of 0.3 and 0.36, the second
leg which is northwest bound has a volume-to-capacity ratio of 0.24 and 0.29, the third leg which
is the southwest bound has a volume-to-capacity ratio of 0.28 and 0.49, and the fourth leg which is
the southeast bound has a volume-to-capacity ratio of 0.17 and 0.2. These results implicate that the
roundabout is surveying below its effective capacity. The values shown first are degree of
saturation in the upper bound and later are degree of saturation in the lower bound.
4.1.3 Delay Calculation and Determination of Level of Service
Delay is a standard parameter used to measure the performance of an intersection. The HCM
identifies control delay as the primary service measure for roundabouts, with level of service
determined from the control delay estimate. As stated earlier the equation below shows the model
that is used to estimate delay for each lane of a roundabout approach.
3,600
(
) 𝑥𝑖
3,600
√
𝑐𝑖
𝑑𝑖 =
+ 900𝑇 [𝑥𝑖 − 1 + (𝑥𝑖 − 1)2 +
] + 5 ∗ min[𝑥𝑖 , 1]
𝑐𝑖
450𝑇
Earlier in the methodology section the parameters of this section were explained as follows:

Xi - volume-to-capacity ratio of approach i

Ci - capacity of approach i

di - average control delay for approach i (seconds/vehicle)

T - analysis time period (hour)
54
In the following computations the greater value of volume-to-capacity of approach in the upper
bound and lower bound were used since the worst case scenario for design and performance
analysis purposes should be considered. Likewise the smaller values of capacity of each approach
in the upper bound and lower bound were used, since smaller capacity reduces results in
performance and hence level of service. Therefore, the results from the lower bounds of both
capacity and volume-to-capacity ratio were taken. These results will be used later in the comparison
and conclusion section.
Northeast direction
3,600
𝑑𝑛𝑒 =
+ 900(0.25) [0.36 − 1 + √(0.36 − 1)2 +
997
(
3,600
)0.36
997
450(0.25)
] + 5 ∗ min[0.36,1]=7.43s/veh
Northwest direction
𝑑𝑛𝑤 =
3,600
867
+ 900(0.25) [0.29 − 1 + √(0.29 − 1)2 +
(
3,600
)0.29
867
450(0.25)
] + 5 ∗ min[0.29,1]=7.29s/veh
Southwest direction
𝑑𝑠𝑤 =
3,600
686
+ 900(0.25) [0.49 − 1 + √(0.49 − 1)2 +
(
3,600
)0.49
686
450(0.25)
]+5∗
min[0.49,1]=12.63s/veh
Southeast direction
𝑑𝑠𝑒 =
3,600
847
(
3,600
)0.2
847
+ 900(0.25) [0.2 − 1 + √(0.2 − 1)2 + 450(0.25)
] + 5 ∗ min[0.2,1]=6.31s/veh
Delay for a given lane is a function of the lane’s capacity and degree of saturation. The analytical
model used above to estimate delay assumes that there is no residual queue at the start of the
analysis period. If the degree of saturation is greater than about 0.9, the delay is significantly
affected by the length of analysis period. In most case, the recommended analysis period is 15
minutes. If demand exceeds capacity during a 15-minute period, the delay results calculated by the
procedure may not be accurate due to the likely presence of a queue at the start of the time period.
In addition, the conflicting demand for movements downstream of the movements downstream of
55
the movement operating over capacity may not be fully realized (in other words, the flow cannot
get past the oversaturated entry and thus cannot conflict with a downstream entry).
The first leg which is the northeast bound has a delay of 7.43seconds per vehicle, the second leg
which is northwest bound has a delay of 7.29seconds per vehicle, the third leg which is the
southwest bound has a delay of 12.63seconds per vehicle, and the fourth leg which is the southeast
bound has a delay of 7.43seconds per vehicle. These results implicate that northeast, northwest,
and southeast bounds have delay under 10 seconds, while the southwest bound has a delay in the
range 10-15. In addition to that the degree of saturation of each leg is under 1.0.
The HCM defines LOS as a quantitative stratification of a performance measure or measures that
represent the quality of service. For roundabouts, LOS has been defined using delay with criteria
given in table 3.11. From the table LOS F is assigned if the degree of saturation of a lane exceeds
1.0 regardless of the control delay. Since all legs have a degree of saturation below 1.0 we use only
control delay as a quality of measure. According to HCM northeast, northwest, and southeast
bounds have LOS A; and southwest bound has a level of service of LOS B.
4.2
Software Analysis
Based on the input from geometric and traffic data of Goduguada roundabout SIDRA 8.0
performance analysis software produced the following results. The performance analysis
results for the roundabout are summarized in Table 4.3. The performance is measured
with v/c ratio or degree of saturation and level of service also applied according to HCM
manual. Detailed analysis results of SIDRA are available in Appendix B.
Environmental factor of 1.0 and lane utilization ratio of 100 have been used in this analysis.
Environment factor is a parameter used to calibrate the roundabout capacity model to allow for less
restricted (higher capacity) and more restricted (lower capacity) roundabout environments. Higher
capacity conditions could be a result of factors such as good visibility, more aggressive and alert
driver attitudes (smaller response times), smaller vehicles in the vehicle population, negligible
pedestrian volumes, and insignificant parking and heavy vehicle activity (goods vehicles, buses,
trams stopping on approach roads). Lower capacity (more restricted) conditions could be a result
of factors such as compact roundabout design (perpendicular entries), low visibility, relaxed driver
attitudes (slower response times), large vehicles in the population, high pedestrian volumes, and
significant parking and heavy vehicle activity (goods vehicles, buses, trams stopping on approach
56
roads). (SIDRA User Guide, April 2008). Lane utilization ratio is the distribution of vehicles
among lanes when two or more lanes are available for a moment.
Table 4.3. Summarized performance analysis results on the roundabout
Roundabout
Total
Heavy
Effective
Average
Degree of
Level of
Vehicle
Vehicles
Capacity
Control
Saturation
Service
Flow
(%)
(veh/h)
Delay
(v/c)
(veh/h)
Goduguada
1145
(sec)
4.9
2866
6.9
0.4
LOS A
4.2.1 Movement Summary
As we can see from the table below (Table 4.4) most of the lanes are in free flow condition and
overall it shows free flow condition where traffic flow is virtually zero. All lanes of each leg have
a level of service of LOS A with the exception of the origin-destination movements of ArabseferMekuriyaye and Campus-Southstar which have a level of service of LOS B or reasonably free flow
condition.
57
Table 4.4. Summarized movement performance results on each lane
Movement
Turn
ID
Demand Flow
Total
Heavy
Vehicles
Vehicles
(veh/h)
(%)
Capacity
Degree of
Average
Level of
(veh/h)
saturation
Delay
Service
(v/c)
(sec)
Northeast: Mekuriyaye
V1
R2
77
8.3
194
0.399
5.6
A
V2
T1
241
4.0
604
0.399
5.9
A
V3
L2
44
0.0
110
0.399
9.7
A
362
4.5
0.399
6.3
A
Approach
Northwest: Arabsefer
V4
R2
85
7.6
769
0.110
6.3
A
V5
T1
78
0.0
396
0.198
6.4
A
V6
L2
78
2.7
396
0.198
10.3
B
242
3.6
0.198
7.6
A
Approach
Southeast: Southstar
V7
R2
45
4.8
113
0.400
5.6
A
V8
T1
260
5.4
651
0.400
5.9
A
V9
L2
53
10.2
132
0.400
9.8
A
358
6.0
0.400
6.4
A
Approach
Southwest: Campus
V10
R2
45
4.8
738
0.061
6.5
A
V11
T1
81
6.7
531
0.187
6.7
A
V12
L2
57
3.8
305
0.187
10.6
B
183
5.3
0.187
7.9
A
Approach
4.2.2 Capacity
In the intersection summary of SIDRA outputs section the total demand flow of the roundabout is
1144veh/h and the effective capacity of the roundabout is 2866veh/h. This results show that the
roundabout is in free flow conditions and the traffic flow is virtually zero. This implicates that the
overall level of service of the roundabout is LOS A.
58
More over there is one variable called the practical spare capacity which is the amount of increase
possible in the demand flow rate to obtain a degree of saturation equal to the practical (target)
degree of saturation. The amount of practical spare capacity in this case is 112.8% for the
roundabout, which implicates that the roundabout is comfortable and in convenient level for all
road users. In addition to that it can yet survey more traffic to be specific more than two times it is
surveying currently. Table 4.5 summarizes the differences between demand flow and capacity of
the roundabout. This table illustrates that the roundabout’s demand flow is much lesser than the
capacity of the roundabout which implies that, it is in a very comfortable and convenient condition.
Table 4.5. Total capacity per movement vs Demand flow of Goduguada Roundabout
Approach leg
(veh/h)
Flow
(veh/h)
Capacity
Demand
Vehicles
Menaheria
Atote
Weldeamanuel
Shell
Total
V1
V2
V3
V4
V5
V6
V7
V8
V9
V10
V11
V12
77
241
44
85
78
78
45
260
53
45
81
57
1144
194 604 110 769 396 396 113 651 132 738 431 305 4839
4.2.3 Degree of Saturation
Ratio of Demand Volume to Capacity (v/c) for all movement classes
The figure below shows the degree of saturation of each legs of the roundabout based on the
circumstances such as the geometry of the roundabout. It represents the approximate values of
volume-to-capacity ratio to attain a target level of service of LOS A using HCM 2010 Model.
Approaches
Southeast Northeast Northwest Southwest
Degree of Saturation 0.19
0.40
0.20
0.40
Intersection
0.40
59
Color code based on Degree of Saturation
[<0.6]
[0.6-0.7]
[0.7-0.8]
[0.8-0.9]
[0.9-1.0]
[>1.0]
SIDRA INTERSECTION 8.0 ɪ Copyright © 2000-2018 Akcelik and Associates Pty Ltd ɪ sidrasolutions.com
Organization: Fenerbahce Processed: Friday, November 20, 2020 9:51:05 PM
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Figure 4.1. Degree of Saturation of Goduguada Roundabout
4.2.4 Delay
Average control delay per vehicle
The figure below shows the delay on each leg of the roundabout in seconds. All lanes of each leg
have a delay under 10 seconds with the exception of the origin-destination movements of
Arabsefer-Mekuriyaye and Campus-Southstar which have a delay in the range 10-15. The two
lanes have a level of service of LOS B and all other lanes have a level of service of LOS A
according to the legend. But leg and roundabout wise the level of service is LOS A.
60
Approaches
Southeast Northeast Northwest Southwest
Delay
(Control)
LOS
Intersection
7.9
6.3
7.6
6.4
6.9
A
A
A
A
A
Color code based on Level of Service
[LOS A]
[LOS B]
[LOS C]
[LOS D]
[LOS E]
[LOS F]
Site Level of Service (LOS) Method: Delay (SIDRA). Site LOS Method is specified in the Parameter Settings dialog (Site tab).
NA (TWSC): Level of Service is not defined for major road approaches or the intersection as a whole for Two-Way Sign Control (HCM LOS
rule).
Roundabout Level of Service Method: SIDRA Roundabout LOS
SIDRA Standard Delay Model is used. Control Delay includes Geometric Delay.
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Figure 4.2. Control delay of Goduguada Roundabout
4.2.5 Level of Service Summary
Lane Level of Service
The figure below shows the overall performance of each lanes of the roundabout in terms of level
of service. It shows the level of service of each leg on the roundabout. From the figure we can
observe that each leg is in the best operating conditions. All lanes in the roundabout have a level
of service of LOS A.
61
Approaches
Southeast Northeast Northwest Southwest
LOS
A
A
A
A
[LOS A]
[LOS B]
Intersection
[LOS C]
A
[LOS D]
[LOS E]
[LOS F]
Site Level of Service (LOS) Method: Delay (SIDRA). Site LOS Method is specified in the Parameter Settings dialog (Site tab).
Roundabout LOS Method: SIDRA Roundabout LOS.
Lane LOS values are based on average delay per lane.
Intersection and Approach LOS values are based on average delay for all lanes.
SIDRA Standard Delay Model is used. Control Delay includes Geometric Delay.
SIDRA INTERSECTION 8.0 | Copyright © 2000-2018 Akcelik and Associates Pty Ltd | sidrasolutions.com
Organization: Fenerbahce | Processed: Friday, November 20, 2020 9:51:05 PM
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Figure 4.3. Level of Service Summary of Goduguada Roundabout
4.3
Comparison on Manual Analysis and Software Analysis
The main objective of this research is to perform the performance evaluation of Goduguada
Roundabout. To carry out the performance of the roundabout two methods were used (manual
calculations and software analysis) as mentioned on the objectives section. One of the objectives
of this research is to do comparison on the results of the two methods since differences on results
are likely to occur between the two methods. This differences occurred due to the number of
62
parameters used in each of the methods. For instance, performance measure for SIDRA is delay
for all the analysis carried out whereas in the manual calculations degree of saturation, degree of
saturation and level of service are used as performance measures for capacity and delay analysis
respectively. On the software the level of service target used was LOS A and the level of service
method (although it does not affect the network analysis) used was also LOS A.
On the manual analyses different criteria for the performance measures were used. For capacity
vehicle to capacity ratio was used, for delay analysis level of service and velocity to capacity ratio
were used.
The following sections compare results of the two methods.
4.3.1 Traffic Movement
Traffic data is the most significant data for this research as mentioned earlier. Since it is an input
parameter it is not really necessary to make comparison on the movement data. Nevertheless, the
comparison based on video interpretations and input reports from the software comparisons were
made.
When the manual analysis was carried out, the procedures mentioned on section 3.3.3.1 were used
and the summarized results on table 4.2 were obtained. The movement types for each legs of the
roundabout were identified from the recorded video. This summarized data was used in both the
manual and software analyses. When this volume data was used on the software as input it gives
output in the input reports section and from this same data and it appears that there were 21pcu
more on the software. This may be due to differences in decimal places used when computing
manually. Southstar approach leg is the leg where the considerable differences have been observed.
There are other legs where difference were encountered such as Campus and Arabsefer.
Mekuriyaye approach leg has not shown significant differences. Summarized volume data both
from the software and the manual analyses for each movements are presented on table 4.6.
63
Table 4.6. Traffic Volume from the software and count
Approach leg
V2
Southstar
V3 V4 V5 V6 V7
V8
Campus
Total
V9 V10 V11 V12
(pcu/hr)
V1
Arabsefer
77 241 45 95
79 78
46 237 53
46
70
56
1123
(pcu/hr)
Count
Analysis
Software
Traffic
Vehicles
Mekuriyaye
77 241 44 85
78 78
45 260 53
45
81
57
1144
4.3.2 Capacity Evaluation
The results obtained from the software show free flow conditions when compared to demand flows
and virtually zero traffic movement on all legs of the roundabout. In the manual analyses section,
the capacity computation is used to determine the degree of saturation of each leg. Moreover, it
uses only the traffic volume for the analysis. But the software uses the geometry of the roundabout
in addition to the traffic data for analysis purpose.
The HCM uses upper bound and lower bound theorem to compute capacity although the lower
bound for this analysis purpose was chosen. The results on the roundabout from manual and
software analyses are presented in the table below.
Table 4.7. Capacity Evaluation Result
Manual Analyses Software Analysis
Northeast
997veh/h
908veh/h
Northwest
867veh/h
1561veh/h
Southwest
686veh/h
896veh/h
Southeast
847veh/h
1574veh/h
From the table above relatively slight differences can be observed on Mekuriyaye approach leg but
on the other legs there are considerably huge differences. This is due to the differences of input
data used in each analysis (i.e, manual and software analyses).
4.3.3 Degree of Saturation
On the manual analyses section, the computed capacity and approach volume were used to compute
the volume-to-capacity ratio. In fact degree of saturation is also used as performance measure of
64
the capacity of the roundabout. The results obtained from the manual analysis show that the degree
of saturation on each leg were under 1.0 which is below the maximum limit for volume-to-capacity
ratio. As mentioned earlier the manual analysis takes into consideration only the traffic conditions
of the roundabout. On the other hand, the software uses the geometric conditions and other
parameters mentioned above in addition to the traffic conditions of the roundabout although the
values of the volume-to-capacity ratio on each leg were also below 1.0. There are significant
differences in the results between the two methods on Arabsefer, Southstar, and Campus approach
legs which is due to the differences of parameters used in the two methods. On Mekuriyaye
approach leg the differences in the results are not significant. The results from the software and
manual analyses are summarized in the table below.
Table 4.8. Degree of Saturation result from the Manual and Software analysis
Approach Legs
Mekuriyaye Arabsefer Southstar Campus Roundabout
Manual analysis (v/c)
0.36
0.29
0.49
0.29
0.49
Software analysis (v/c)
0.40
0.20
0.40
0.19
0.40
4.3.4 Delay
The values of delay on each leg are in the range where the roundabout performs in the most
comfortable and convenient way. All legs tend to have a free flow condition. All vehicle users on
all legs will only be delayed for a negligible duration of time. The results obtained from the
software are very close to the results obtained from the manual analysis. The table below
summarizes the values of delay obtained from software and manual analyses.
Table 4.9. Delay results from software and manual analysis
Approach Legs
Mekuriyaye Arabsefer Southstar Campus Roundabout
Manual analysis (seconds)
7.43
7.29
12.63
7.43
8.69
Level of Service
A
A
B
A
A
Software analysis (seconds)
6.3
7.6
6.4
7.9
6.9
Level of Service
A
A
A
A
A
4.3.5 Level of Service
The manual analysis determines the level of service of each leg in the quality measures (degree of
saturation, delay, and capacity) analyzed above since each measure gives an output of level of
65
service. In the discussions and analyses made above about these quality measures, there are no
separate analyses made to determine level of service. But in each of the analyses, the level of
service is determined although not directly mentioned on the capacity analysis section. The
software on the other hand gives a summary of level of service for each leg. Table 4.10 gives a
summarized value of level of service on each leg.
Table 4.10. Level of Service results from different manual analyses and software analysis
Approach Legs
Degree of
Manual
analysis
Saturation
Delay
Software analysis
Mekuriyaye Arabsefer Southstar Campus Roundabout
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
66
5
CONCLUSION AND RECOMMENDATION
5.1
CONCLUSION
Based on the results obtained from the research the degree of saturation is 0.40 on software analysis
and 0.49 on manual analysis. This implicates that the roundabout is in free flow conditions. It is
known that for a better performance the degree of saturation of the roundabout should be less than
0.85. Hence, the performance of the roundabout is comfortable and convenient. Likewise, the
average delay of vehicles incurred at the roundabout is about 6.9s/veh on the software analysis and
8.69s/veh on the manual analysis which categorizes it under level of service group of LOS A. There
were also other performance measures which were used to determine the performance of the
roundabout.

Capacity: results from both manual and software analysis are below the roundabout’s
capacity and hence the roundabout is performing in the best possible conditions. The results
from the manual calculation and the software analysis were 1123 veh/h and 1144veh/h
respectively.

Level of service: this performance measure differs from the other quality measures in such
a way that it can be a final performance measure for the other quality measures like delay,
degree of saturation, and capacity. All the results from the other quality measures that were
used to determine level of service both on the manual calculations and software analysis
were all in the level of service group LOS A. Moreover, on the software analysis the
summarized level of service result shows that the roundabout is in free flow condition.
It is therefore concluded that Goduguada roundabout is in free flow conditions and operating in the
best conditions. Based on actual field conditions, it is common to see that at peak hours, the traffic
police does not have to regulate the traffic at the roundabouts or introduction of signals on the
roundabout is not necessary since the introduced roundabout facility is functioning properly or
regulating the traffic in the best possible manner.
67
5.2
RECOMMENDATION
According to the statement of the problem of the research, when we first started this research we
aimed at checking whether or not enough research had been conducted to objectively determine
the efficiency of roundabouts to signalized intersections on Goduguada junction. According to the
results obtained from the software and manual analysis this question seems to be perfectly
answered. These conditions will be maintained the same if and only if the demand traffic increases
in the expected growth rate. If the growth rate tends to increase exponentially or so, the roundabout
may not be able to encompass the traffic in such a way it is doing right now.
It is therefore recommend that Goduguada roundabout can be used as an example for other
junctions with lower traffic volumes. Since uncongested (free flow condition) roundabouts like this
one provide road users with fewer conflicting traffic, and reduced number of traffic accidents; they
are highly recommended for such types of junctions. This types of roundabouts are recommended
also for future traffic demand growth with changeable patterns.
68
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Rodergerdts, L, M Blogg, E. Wemple, E. Myers, M. Kyte, M. Dixon, G. List, A. Flannery,
R. Troutbeck, W. Brilon, N. Wu, B. Persaud, C. Lyon, D. Carter. Roundabouts in the United
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Research Board, National Academics of Science, Washington, D.C., 2007.

Russell Eugene, Margaret Rys and Greg Luttrell, Modelling Traffic Flows and Conflicts at
Roundabouts, MBTC FR-1099, Mav-Blackwell National Rural Transportation Study
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
Simon, M. J. 1991. Roundabouts in Switzerland: Recent Experiences, Capacity, Swiss
Roundabout Guide. Proc., Intersections without Traffic Signals II: Proceedings of an
International Workshop, Springer-Verlag, 41-52.
71

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Kreisverkehrsplaetzen. (Investigation of capacity and safety at German Roundabouts),
Publication of the Institute for Transportation and Traffic Engineering at the RuhrUniversity Bochum. No. 10, 1992

Transportation Research Board, TRB, “Highway Capacity Manual”, National Research
Council, Washington, D.C., 2010.

Taekratok Thaweesak: Modern Roundabouts for Oregon, Oregon, USA, June 1998

Troutbeck, R. 1993. Capacity and Design of Traffic Circles in Australia. Transportation
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
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
The Michigan Department of Transportation, (MDOT): Evaluating the performance and
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72
APPENDICES
Appendix A
Summary of 60-minute light vehicles volume for Goduguada Roundabout
Mekuriyaye-Arabsefer
Mekuriyaye-Southstar
Mekuriyaye-Campus
Arabsefer-Southstar
Arabsefer-Campus
Arabsefer-Mekuriyaye
Southstar-Campus
Southstar-Mekuriyaye
Southstar-Arabsefer
Campus-Mekuriyaye
Campus-Arabsefer
Campus-Arabsefer
1:301:45
13
51
13
23
14
10
7
53
8
4
15
10
1:452:00
15
47
11
19
25
19
5
52
16
11
22
12
2:002:15
17
58
9
17
16
24
14
59
15
13
23
10
2:152:30
21
59
8
14
18
18
14
65
5
12
10
19
PHF
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
Total Raw
Vehicles
66
215
41
73
73
71
40
229
44
40
70
51
Total
Vehicles
71
231
44
78
78
76
43
246
47
43
75
55
Summary of 60-minute heavy vehicles volume for Goduguada Roundabout
Mekuriyaye-Arabsefer
Mekuriyaye-Southstar
Mekuriyaye-Campus
Arabsefer-Southstar
Arabsefer-Campus
Arabsefer-Mekuriyaye
Southstar-Campus
Southstar-Mekuriyaye
Southstar-Arabsefer
Campus-Mekuriyaye
Campus-Arabsefer
Campus-Southstar
1:301:45
2
2
0
2
0
0
0
2
2
0
0
0
1:452:00
2
3
0
2
0
2
2
8
0
2
0
2
2:002:15
2
2
0
0
0
0
0
0
0
0
3
0
2:152:30
0
2
0
2
0
0
0
3
3
0
2
0
PHF
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
Total Raw
Vehicles
6
9
0
6
0
2
2
13
5
2
5
2
Total
Vehicles
6
10
0
6
0
2
2
13
5
2
5
2
73
Summary of 60-minute traffic volume for Goduguada Roundabout
Mekuriyaye-Arabsefer
Mekuriyaye-Southstar
Mekuriyaye-Campus
Arabsefer-Southstar
Arabsefer-Campus
Arabsefer-Mekuriyaye
Southstar-Campus
Southstar-Mekuriyaye
Southstar-Arabsefer
Campus-Mekuriyaye
Campus-Arabsefer
Campus-Southstar
Light Vehicles
Heavy Vehicles
Total Vehicles
71
231
44
78
78
76
43
246
47
43
75
55
6
10
0
6
0
2
2
14
5
2
5
2
77
241
44
84
78
78
45
280
52
45
80
57
Percentage of Heavy
Vehicles (%)
7.79
4.15
0
7.14
0
2.56
4.44
5
9.62
4.44
2.5
3.51
74
Appendix B
Input Report
75
76
Intersection Summary
Lane Summary
77
Roundabout Analysis
78
Level of Service Summary
Approaches
Southeast Northeast Northwest Southwest
LOS
A
A
A
A
Intersection
A
Color code based on Level of Service
[LOS A]
[LOS B]
[LOS C]
[LOS D]
[LOS E]
[LOS F]
Site Level of Service (LOS) Method: Delay (SIDRA). Site LOS Method is specified in the Parameter Settings dialog (Site
tab).
Roundabout LOS Method: SIDRA Roundabout LOS.
Lane LOS values are based on average delay per lane.
Intersection and Approach LOS values are based on average delay for all lanes.
SIDRA Standard Delay Model is used. Control Delay includes Geometric Delay.
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