TRAFFIC SIGNALS
INTRODUCTION
The travel characteristics, road networks and local constraints are
very different in the cities of developing countries than those of
developed countries. It is therefore necessary to determine the
different parameters of traffic movements which are suitable for
local urban transport system characteristics. One such effort is to
determine passenger car equivalent (PCE) values at signalized
intersections using an established method for Hyderabad
Metropolitan City where the street networks, travel patterns and
local constraints are completely different from any other city in the
world. In the design of a signalized intersection, it is very important
to determine the saturation flow of that intersection. Saturation flow
is the maximum constant departure rate of queues from the stop line
of an approach lane during the green signal period. An idealized view
of saturation flow at signalized intersections is illustrated in Figure .
However, saturation flows in vehicles per hour depend on the type
and proportion of vehicles in the traffic stream2. In most developing
countries, urban traffic flow is heterogeneous in nature. As a result it
is necessary to convert heterogeneous traffic into a stream of
homogenous traffic by using appropriate passenger car equivalents
(PCE) for analyzing mixed traffic. In addition, appropriate PCE are
also used for capacity analysis as well as traffic engineering research.
INDIA is based on the values given in the Geometric Design of
Highways, Ministry of Communication. The values of PCE in the
Geometric design of Highways are the modification of the PCE given
by Webster. These PCE derived in those research are shown in Table
1. The values determined by Webster are based on the study
performed in the United Kingdom in the 50’s and 60’s. But now-a-
days, more powerful vehicles are introduced on the roads through
the use of automatic transmissions and by increasing the power to
weight ratios and also the driver's behavior, traffic compositions and
the roadway characteristics in Bangladesh are far different from that
of the original publication. As a result these values may not be
representative of local traffic conditions in Hyderabad. Furthermore,
the number of vehicles and their compositions in Metropolitan City
is increasing day by day with the increase of population. Statistics
shows that the growth rate of motorized vehicles in INDIA is around
30 percent per annum. Moreover, the compositions of the traffic at
the studied intersections are very complex. Both motorized and nonmotorized traffic moves through the intersections. There is no lane
marking or lane restriction, no phase in the traffic signal for right
turning vehicles, inadequate pedestrian facilities, poor parking
facilities, poor road surface condition and so on. To reduce the
effects of this increasing amount of vehicles and their compositions
on the operational system of the signalized intersections and
considering other factors, it is necessary to determine the PCE on the
basis of current roadway and traffic conditions of Hyderabad
Metropolitan City. Therefore, the objective of this paper is to
determine the PCE that reflects the local traffic conditions in
Metropolitan City. Emphasis was placed on the through movement
of different types of vehicle. The main vehicle compositions observed
during the study consist of passenger cars, auto-rickshaws, minibuses and buses. The performance of the signalized intersections can
be improved by using these PCE.
STUDY APPROACH
Methods commonly used for deriving PCU5 fall into two groups,
specifically the headway ratio method and regression method.
Headway ratio method has been used by Brown & Odgen9, Cuddon
& Odgen10, Hasan et al.11 and William H.K. Lam2 while the
regression method has been used by Branston and van Zuylen12 and
Kimber et al. 1. In this study, the headway ratio method was used for
the calculation of PCE. According to Leong13, the saturation flow
rate based on PCE of the headway ratio method predicts better than
the saturation flow rate based on PCU of regression analysis. This
procedure is recommended by examining the condition — ‘necessary
and sufficient condition for the effect of a certain type of vehicle to
be independent of the type of vehicle preceding it and following it’.
The following condition should be satisfied if it is wanted to calculate
PCE by the headway ratio method10. It involves a com
TRAFFIC SIGNAL DEVICES
• One of the most important and effective methods of controlling
traffic at an intersection
• Electrically time device that assigns the right-of-way to one or
more traffic streams so that these streams can pass through the
intersection safely and efficiently
 Suitable when there are:
▫ Excessive delays at stop signs and yield signs
▫ Problems caused by turning movements
▫ Angle and side collision
▫ Pedestrian accidents
What is the purpose of a traffic signal?
Traffic signals are designed to ensure an orderly flow of traffic,
provide an opportunity for pedestrians or vehicles to cross an
intersection and help reduce the number of conflicts between
vehicles entering intersections from different directions.
Do traffic signals reduce intersection crashes?
Certain types of crashes can be reduced in number or severity by
installing a signal, while other types of crashes will increase. Where
signals are used unnecessarily, the most common result is an
increase in total crashes, especially rear-end collisions.
How do traffic signals work?
Fixed-time signals follow a predetermined sequence of signal
operation, always providing the same amount of time to each traffic
movement, whether traffic is present or not. Actuated signals change
the lights according to the amount of traffic in each direction. They
use various types of sensors to detect vehicles, and adjust the length
of the green time to allow as many vehicles as possible through the
intersection before responding to the presence of vehicles on
another approach.
Advantages of traffic signals
Traffic signals control vehicle and pedestrian traffic by assigning
priorities to various traffic movements to influence traffic flow.
Properly designed, located and maintained traffic signals have one or
more of these advantages:
 Provide for orderly movement of traffic;
Increase traffic-handling capacity of an intersection;
 Reduce frequency and severity of certain types of crashes,
especially right-angle collisions;
 Provide for continuous movement of traffic at a definite speed
along a given route;
 Interrupt heavy traffic at intervals to permit other vehicles or
pedestrians to cross.
 provisions for side-street vehicles to enter the traffic stream,
 provisions for the progressive flow of traffic in a signal-system
corridor,
 possible reductions in delay.
 Control traffic more economically than by manual methods
Disadvantages :
Traffic signals are not a solution for all traffic problems at
intersections, and unwarranted signals can adversely affect the
safety and efficiency of traffic by causing one or more of the
following:
 Excessive delay;
Increased traffic congestion, air pollution and fuel consumption;
Improperly timed, cause excessive delay, increase driver irritation
leads to disobedience of signals;
Increased use of less-adequate streets to avoid traffic signals;
Increased frequency of crashes, especially rear-end collisions.
provisions for pedestrians to cross the street.
 causing a disruption to traffic progression (adversely impacting
the through movement of traffic),
 encouraging the use of routes not intended for through traffic
(such as routes through residential neighbourhoods).
Signal Control Strategies:
Fixed time operation of signals is an unsatisfactory method of
control. Control strategy is usually flexible and is achieved by one of
the following methods:
a) vehicle actuation: a series of buried loops are placed on the
approaches with the initial detector about 40m before the stop line.
b) Cable-less linking of signals: using the coordination between
signals.
c) Cable linked system: This is an older system where cables are
connected between controllers of traffic signals.
d) Integral time switch: using a central computer. This method can
control a wide area.
Required studies for Traffic Signals:
The decision to install a traffic signal should be based on a through
investigation. The required studies to gather the necessary data
include:
Traffic volume studies:
Approach travel speed:
Physical conditions diagram:
Accident history and collision
diagram:
Gap studies:
Delay studies
Traffic and pedestrian counts
Spot speed studies.
Geometric, channelization,
grades, sight distance, bus stops,
parking conditions, road furniture
and land use.
Over a year including type of
collision, vehicle type, time,
severity, lighting conditions.
Weather conditions
In the major road traffic
INDIAN PRACTICE
Basic Timing Elements
Elements within a phase:
– Green interval: the period of the phase during which the green
signal is illuminated.
– Yellow/amber interval: the portion of the phase during which the
yellow light is illuminated.
– All-red interval: the period during which the red light is illuminated
for all approaches
– Inter-green interval: the interval between the end of green for one
phase and the beginning of green for another phase
• Red time: The time period in which the traffic signal has the red
indication
• Cycle: One complete rotation or sequence of all signal indications
[green, yellow, red]
• Cycle time (or cycle length): The total time for the signal to
complete one sequence of signal indication. The time interval
between start of a green till next green for a any approach
• Cycle time maximum (Cmax ) is 120 second is considered as good
practice. Normally, the cycle time will lie within the range of 30 – 90
s.
• Interval: Indicates the change from one stage to another. There are
two types of intervals
–change interval
–clearance interval.
• Change interval is also called the yellow time indicates the interval
between the green and red signal indications.
• Clearance interval is also called all red is included after each yellow
interval indicating a period during which all signal faces show red
• It is used for clearing of the vehicles in the intersection
Change interval:
Identical to the intergreen interval (the time between the end of a
green indication for one phase and the beginning of a green
indication for another)
This yellow sign indicates the change interval time
Clearance interval:
Identical to the all red interval (the display time of a red indication
for all approaches)
Definitions and notations
• Phase ▫ Green interval + change interval + clearance interval
▫ During green interval non-conflicting movements are cleared
• Lost time
▫ Time during which intersection is not effectively utilized for any
movement
▫ E.g. reaction time of the first driver in the queue
• Phase: A phase is the green interval plus the change and clearance
intervals that follow it. Thus, during green interval, non conflicting
movements are assigned.
• It allows a set of movements to flow and safely halt the flow
before the phase of another set of movements start
Phase
Green, yellow, and all-red time provided for one movement. The sum
of the displayed green, yellow, and red times for a movement or
combination of movements that receive the right of way
simultaneously during the cycle. The sum of the phase lengths (in
seconds) is the cycle length.
CYCLE LENGTH
Interval
An individual signal indication (green,yellow, or red) within a phase
Inter-green period:
The period between one phase losing right of way and the next
phase gaining right of way is known as the inter-green period.
In other words it is the period between the termination of green on
one phase and the commencement of green on the next phase,
Inter-green period
Examples of inter-green periods at a two-phase traffic signal as
shown below.
Definitions
• Cycle: One complete color changes sequence
Made up of phases
• Cycle Length: Time (sec) needed for one cycle completion
• Phase:
Red, Green, Yellow, Left Turn
Part of a cycle that is allocated to one or more movements
• Effective Green (g): Time when vehicle can enter the intersection
Green + Part of Yellow
• Effective Red (r): Time (sec) not effectively used by an approach for
traffic movement
• Lost time (LT)
- Total time in one cycle when nobody can enter the intersection
from any approach
– Every time phases change there is some lost time
– All red+ part of green and yellow
• Change Interval (clearance interval)
– Yellow + All red
– Time (sec) to clear the intersection after the green interval before
the conflicting movements are released
• Coordination –
Timing of the signals so that a "platoon" of cars proceeds through
multiple intersections without stopping
• Offset –
Time difference between green signals in successive intersections
Saturation Flow Rate
• The saturation flow rate is the maximum hourly volume that can
pass through an intersection, from a given lane or group of lanes, if
that lane (or lanes) were allocated constant green over the course of
an hour
. s  3600/ h
Where;
• s = saturation flow rate in veh/h,
• h = saturation headway in s/veh,
• 3600 = number of seconds per hour.
Saturation Flow Rate
• Factors that can affect the maximum flow rate through an
intersection:
• lane widths,
• grades,
• curbside parking
• distribution of traffic among multiple approach lanes,
• the level of roadside development,
• bus stops,
• influence of pedestrians, bicycles, and heavy vehicles
Rate of flow against time
• Soon as green signal is given the rate of discharge begins to pick up
and some time the time is lost before the flow reaches the maximum
value (saturated slow)
• At the terminal of green phase the flow tends to taper off involving
the lost time
Effective Green Time
The time during which the flow is assumed to take place atsaturation
flow
Effective GreenTimes
• For analysis purposes, the time during a cycle that is effectively
utilized by traffic must be used (the green, yellow, and red signal
indications are not directly useful for analysis).
• Effective green time is the time during which a traffic movement is
effectively utilizing the intersection.
g  G  Y  AR tL
Where:
– g = effective green time for a traffic movement in seconds,
– G = displayed green time for a traffic movement in seconds,
– Y = displayed yellow time for a traffic movement in seconds,
– AR = displayed all-red time in seconds, and
– tL = total lost time for a movement during a cycle in seconds.
Phase design
• The signal design procedure involves six major steps
. • They include the
– phase design, – determination of amber time and clearance time,
– determination of cycle length,
– apportioning of green time,
– pedestrian crossing requirements, and
– the performance evaluation of the above design.
Two phase signals
• Two phase system is usually adopted if through traffic is significant
compared to the turning movements.
• Traffic flow 3 and 4 are grouped in a single phase
• Traffic flow 1 and 2 are grouped in the second phase.
• Flow 5,6,7 and 8 offer some conflicts and are called permitted right
turns.
• Needless to say that such phasing is possible only if the turning
movements are relatively low.
• if the turning movements are significant ,then a four phase system
is usually adopted.
Four phase signals
• Flow from each approach is put into a single phase for avoiding all
conflicts.
• The turning movements are comparable with other movements
• When through traffic and turning traffic need to share same lane.
• This phase plan could be very inefficient when turning movements
are relatively low.
Signal Types
- Fixed-Time
- Semi-Actuated
–Fully Actuated
- Volume Density
Types of Traffic Signals
• Pretimed:
• Fixed interval lengths in fixed sequence
• Repeat a preset constant cycle.
• Actuated:
▫ Respond to the presence of vehicles and pedestrians
▫ Need to use in conjunction with vehicle detectors
• Semi-actuated (traffic-adjusted) Predefined timing schemes
selected based on traffic flow information
▫ Detectors placed only on the minor approach
▫ Major approach is interrupted only if vehicle present at the minor
approach
• Fully actuated
▫ Detectors are installed at all approaches
▫ Green time are allocated based on the incoming traffic on each
approach
Fixed Time Signals
• Run the same cycle and phase splits regardless of traffic volumes
• Cycle length and phase times are fixed
• No detectors
Fixed-Time Signals Pros & Cons
• Pros:
• Easily programmed
• No detector maintenance
• Easily coordinated
• Cons:
• Not responsive to traffic
• Can increase delays during offpeak periods
Fixed-Time Signals Common Uses
– Urban areas with dense signal networks
- High pedestrian activity areas
- Intersections with predictable, fairly constant traffic patterns
Webster’s Method
• Most prevalent
• Minimizes intersection delay
Co = Optimum cycle length (sec)
L = Total lost time per cycle, usually taken as the sum of the total
yellow and all-red intervals (sec) (i.e. total intergreen intervals)
Yi = Ratio of the observed flow rate (in straight-through passenger
cars per hour) to the saturation flow rate for the critical approach or
lane in each phase
PASSENGER CAR UNIT (PCU)
Passenger Car Unit (here in after referred as PCU) is a vehicle
unit or car unit used to measure the rate of traffic flow on
highway. In other words, PCU is a measure of number of vehicles
moving on a highway at a given point of time. In some instances,
PCU is referred to as Passenger Car Equivalent (PCE).
Traffic flow is a measure of flow of vehicles on a road from one
point to another point at a given point of time. The flow of traffic
on highway consists of different classes of vehicles that are
classed as follows:
1.
2.
3.
4.
5.
Cars
Buses
Heavy Commercial Vehicles
Light Commercial Vehicles
Auto-Rickshaws
6. Two-wheeler automobiles etc..
The movement of different class of vehicles of highway is termed
as mixed traffic flow. In developing countries like India, the traffic
stream consists of mixed traffic flow.
METHODS OF ESTIMATING PCU
The different types of methods mostly used to determine the
different vehicle classes of PCU values are:
1. Homogenous Coefficient Method
2. Headway method
3. Multiple Linear Regression Method
4. Walker’s Method
5. Simulation Technique
6. Simultaneous Equations Method
7. Huber Method
8. Speed Based Method
FACTORS AFFECTING PCU VALUES
Several factors influence the PCU values of different vehicle
types. such as
1. Dimensions of vehicle i.e. Length and width
2. Dynamic characteristics of the vehicle – speed, acceleration,
etc.
3. Environmental and Climatic conditions
4. Traffic regulation and control
5. Roadway characteristics such as gradient, curves,
intersections, geometrics, etc.
6. Characteristics of traffic stream
7. There should be enough space between the moving vehicles.
The PCU value of a specific vehicle may not be as consistent as
commonly believed. The PCU value of a vehicle class is found to
fluctuate based on a number of factors. As a result, the PCU of a
certain vehicle class is a dynamic changeable value that rarely
remains constant.
SATURATION FLOW RATE
The saturation flow rate, s, is an important parameter
for estimating the performance of a particular
movement. Saturation flow rate is simply the headway
in seconds between vehicles moving from a queued
condition, divided into 3600 seconds per hour. For
example, vehicles departing from a queue with an
average headway of 2.2 seconds have a saturation flow
rate of 3600 / 2.2 = 1636 vehicles per hour per lane.
Saturation flow rate for a lane group is a direct function
of vehicle speed and separation distance. These are in
turn functions of a variety of parameters, including the
number and width of lanes, lane use (e.g., exclusive
versus shared lane use, aggregated in the HCM as lane
groups), grades, and factors that constrain vehicle
movement such as presence or absence of conflicting
vehicle and/or pedestrian traffic, on-street parking, and
bus movements. As a result, saturation flow rates vary
by movement, time, and location and commonly range
from 1,500 to 2,000 passenger cars per hour per lane
(2). The HCM provides a series of detailed techniques
for estimating and measuring saturation flow rate. It
should be noted that this is significantly different than
the ideal saturation flow rate, which is typically
assumed to be 1,900 passenger cars per hour per lane.
The ideal saturation flow rate may not be achieved
(observed) or sustained during each signal cycle. There
are numerous situations where actual flow rates will
not reach the average saturation flow rate on an
approach including situations where demand is not able
to reach the stop bar, queues are less than five
vehicles in a lane, or during cycles with a high
proportion of heavy vehicles. To achieve optimal
efficiency and maximize vehicular throughput at the
signalized intersection, traffic flow must be sustained at
or near saturation flow rate on each approach. In most
HCM analyses, the value of saturation flow rate is a
constant based on the parameters input by the user,
but in reality, this is a value that varies depending on
the cycle by cycle variation of situations and users.
The HCM provides a standardized technique for
measuring saturation flow rate. It is based on
measuring the headway between vehicles departing
from the stop bar, limited to those vehicles between
the fourth position in the queue (to minimize the effect
of startup lost time) and the end of the queue. The
detailed procedure can be found in Chapter 16 of the
HCM. For signal timing work, it is often not necessary
to place heavy emphasis on this parameter due to the
high degree of fluctuation in this parameter from cycle
to cycle.
CLASSIFICATION OF TRAFFIC SIGNALS
Traffic signals are the control devices which alternately direct
the traffic to stop and proceed at intersections using red and
green traffic light signal automatically.
The signals are classified into the following types:






Traffic Control Signals
Fixed time signals
Manually operated signals
Traffic actuated (automatic) signals
Pedestrian signals
Special traffic signals
HOW TO CLASSIFY TRAFFIC SIGNALS?
Suryakanta | August 28, 2015 | Transportation | No Comments
CLASSIFICATION OF TRAFFIC SIGNALS
Traffic signals are the control devices which alternately direct the traffic to stop and proceed at
intersections using red and green traffic light signal automatically.
The signals are classified into the following types:






Traffic Control Signals
Fixed time signals
Manually operated signals
Traffic actuated (automatic) signals
Pedestrian signals
Special traffic signals
1.TRAFFIC CONTROL SIGNALS
Traffic signal
These are provided with three colored light glows facing each
direction of traffic flow.
Red light indicates STOP
Yellow amber light indicates the clearance time for the vehicles
which have entered the intersection area by the end of green
signal
Green light indicates GO
A typical traffic signal showing the arrangement of three light
glows is shown in this fig.
Traffic control signal are further classified into the following 3
types.
A.FIXED TIME SIGNALS
These signals are set to repeat regularly a cycle of red, amber
yellow and green lights. Depending upon the traffic intensities,
the timings of each phase of the cycle is predetermined. Fixed
time signals are the simplest type of automatic traffic signals
which are electrically operated.
Draw backs of the signals: The cycle of red, yellow and green
goes on irrespective whether on any road, there is any traffic or
not. Traffic in the heavy stream has to stop at end phase.
B.TRAFFIC ACTUATED SIGNALS
In these signals the timings of the phase and cycle are changed
according to traffic demand.
In semi-actuated signals, the normal green phase of a traffic
stream may be extended upto a certain period of time for
allowing the vehicles to clear off the intersection.
In fully-actuated signals, computers assign the right of way
for the traffic movement on turn basis of traffic flow demand.
C.MANUALLY OPERATED SIGNALS
In these types of signals, the traffic police watches the traffic
demand from a suitable point during the peak hours at the
intersection and varies the timings of these phases and cycle
accordingly.
2.PEDESTRIAN SIGNALS
When the vehicular traffic remains stopped by red or stop
signal on the traffic signals of the road intersection, these
signals give the right of way of pedestrians to cross a road
during the walk period.
3.SPECIAL SIGNALS OR FLASHING BEACONS
These signals are used to warn the traffic.
When there is a red flashing signal, the drivers of vehicles must
stop before entering the nearest cross walk at the intersection
or at a stop line where marked.
Flashing of yellow signals are used to direct the drivers of the
vehicular traffic to proceed with caution.
Types of signals
Vehicle Actuated Controlled (VAC)
The full form of VAC is vehicle actuated controlled system. This
means that the traffic lights are actuated by the presence of
vehicles( more vehicles mean more time for green signal for
that lane). This is opposed to the fixed time or FT system where
irrespective of the volume of vehicles in a lane, the traffic lights
change after a fixed time automatically.
Advanced Transportation Controller (ATC)
The Advanced Transportation Controller (ATC) Standards are
intended to provide an open architecture hardware and
software platform that can support a wide variety of Intelligent
Transportation Systems (ITS) applications including traffic
management, safety, security and other applications. The ATC
Standards are being developed and maintained under the
direction of the ATC Joint Committee (JC) which is made up of
representatives from the American Association of State
Highway and Transportation Officials (AASHTO), the Institute
of Transportation Engineers (ITE), and the National Electrical
Manufacturers Association (NEMA).
The Advanced Transportation Controller Standard (ATC
Standard) is one of the four standards of the ATC family of
standards. The ATC Standard provides a powerful, on-street,
computing platform for numerous ITS applications intersection
control, ramp metering, data collection, safety, security and
other applications. It uses a transportation controller
architecture where the computational components of the
controller reside on a single small printed circuit board (PCB),
called the “Engine Board,” with standardized connectors and
pinout. It is made up of a central processing unit (CPU), Linux
operating system (O/S), memory, external and internal
interfaces, and other associated hardware necessary to create
an embedded transportation computing platform. The Engine
Board plugs into a “Host Module” which supplies power and
physical connection to the input/output (I/O) facilities of the
controller. While the interface to the Engine Board is completely
specified, the Host Module may be of various shapes and sizes
to accommodate innumerable transportation controller designs
and cabinet architectures. The ATC Standard works together
with the Application Programming Interface (API) Standard to
allow application programs to share the resources of the
controller and the transportation field cabinet system.



ATC Controller Working Group
ATC Joint Committee and Interested Parties
Join the Standards discussion groups on ITE Community
multinomial logit model

Urban expressway interchanges have become accidentprone sites owing to the accelerated increase in motorvehicle ownership. This study explored the impact of
factors, including day of the week, time of day, congestion
level, traffic control devices, and road conditions, on road
safety risk levels in the interchange area of an urban
expressway based on aggregate driving behavior
data. Method: A large amount of aggregate driving
behavior data were obtained from AutoNavi navigation
software. The database was built by matching various
types of data and observing their characteristics. Day of
the week, time of day, congestion level, road conditions
(number of lanes, traffic disturbance, and traffic control


devices [the type of advance guide sign system, number
of warning signs, and the complexity of the diagrammatic
guide sign]) were identified as the explanatory variables.
The traffic order index (TOI), based on driving behavior
and speed variation, was used to evaluate the road safety
risk levels, including risky roads, general roads, and safe
roads, which served as the response variables. The
multinomial logit model (MNL) was developed to explore
the impact of various factors, including traffic control
devices and road conditions, on road safety risk
levels. Results: The results showed that the factors that
significantly influence risky roads include day of the week,
number of lanes, congestion level (slow moving), traffic
disturbance (with the merge or diverge within 500 m), type
of advance guide sign system (three-level advance guide
sign system), and complexity of diagrammatic guide signs
(low or medium complexity). Practical Applications: This
study could offer plausible suggestions for traffic
management departments for the rehabilitation of road
conditions and traffic control devices in urban expressway
interchange areas.
Introduction
The urban expressway is a crucial component of road
networks, and its efficient and safe operation is essential
for the overall traffic operation of the city. By the end of
2017, the Beijing Urban Expressway accounted for 6.1%
of urban roads in Beijing. The speed on urban
expressways is high, and a driver’s delayed reaction to
guide signs can easily cause traffic accidents and traffic
jams. Due to the fact that the traffic flow characteristics of
urban expressways are different from those of other
roads, traffic accidents on urban expressways have a
significant impact on traffic safety. Urban expressway
interchanges, which are a decisive part of the urban
expressway system and play a critical role in alleviating
traffic conflicts, improving traffic capacity, reducing the
incidence of traffic accidents, and improving driving
comfort, can connect either two intersecting expressways
or an expressway with general roads. Beijing has the
largest number of interchanges in China. By the end of
2015, there were approximately 245 interchanges within
the Fifth Ring Road of Beijing. Fig. 1 shows some large
typical interchanges. Therefore, urban expressways and
expressway interchange areas are crucial study areas of
urban road safety improvement. Furthermore, to improve
road safety in the expressway interchange area, it is of
vital importance to explore the factors affecting road safety
risk and to put forward optimization strategies of road
conditions and traffic control devices.

Many studies have focused on accident, traffic conflict,
and driving behavior data to make safety evaluations in
urban expressway interchange areas. First, many studies
have utilized accident data to make safety evaluations in
interchange areas. Chen et al. (Chen et al., 2011)
evaluated the safety performance of left-side off-ramps by
comparing it to that of right-side off-ramps at freeway
diverge areas based on crash records of a total of 11 leftside and 63 similar right-side diverge areas in Florida, and
the results showed that the left-side off-ramp did have
higher average crash counts, crash rate, and percentage
of severe crashes, but the difference is only statistically
significant for the severe crashes at a 10% level. Wang et
al. (Wang et al., 2019) analyzed the real-time crash risk for
expressway ramps using traffic, geometric, sociodemographic, and trip generation predictors based on
real-time crash data. Haleem et al. (Haleem et al., 2013)
described a study to develop crash modification factors
(CMFs) for interchange influence areas on urban freeways
in the state of Florida through multivariate adaptive
regression splines (MARS) for “total” and “fatal and injury”
(FI) crashes based on four years of crashes from 2007 to
2010. Persaud et al. (Persaud and Lyon, 2006) described
the development of new safety performance functions for
interchanges, ramps, and ramp terminals for Ontario
freeways, using negative binomial regression modeling
that related collision frequency to traffic volumes and basic
entity characteristics.

Next, many studies have utilized traffic conflict to make
safety evaluations in the interchange area. Li et al. (Li et
al., 2016) evaluated the traffic safety of freeway
interchange merging areas through the traffic conflict
technique based on field and simulation data, including
the outer lane capacity in the merging area, ramp
capacity, acceleration lane length, percentage of heavy
vehicles on ramps, percentage of heavy vehicles on the
outer lane, and designed ramp speed. Zhou et al. (Zhou et
al., 2011) evaluated the safety condition of the
Zhaoshangang Tunnel and the Xiapu Interchange area in
the Haosifang direction of the highway in Ningbo based on the
traffic volume and road length through the traffic conflict
technique, and found that the proposed method had certain
advantages compared to the traditional evaluation method,
which was based on accident statistics in highway safety
evaluation. To investigate this hazardous condition in the
merging area, Chin and Quek (Chin and Quek, 1997) utilized
the traffic conflict technique to assess the probability of an
occurrence of a serious conflict due to a merging situation on
an expressway.
Peak hour factor
Traffic engineers focus on the peak-hour traffic volume in evaluating
capacity and other parameters because it represents the most critical
time period. And, as any motorist who travels during the morning or
evening rush hours knows, it’s the period during which traffic volume
is at its highest. The analysis of level of service is based on peak rates
of flow occurring within the peak hour because substantial short-term
fluctuations typically occur during an hour. Common practice is to
use a peak 15-minute rate of flow. Flow rates are usually expressed in
vehicles per hour, not vehicles per 15 minutes. The relationship
between the peak 15-minute flow rate and the full hourly volume is
given by the peak-hour factor (PHF) as shown in the following
equation:
If 15-minute periods are used, the PHF is computed as:
Where
V = peak-hour volume (vph)
V15 = volume during the peak 15 minutes of flow (veh/15 minutes)
Typical peak-hour factors for freeways range between 0.80 and 0.95.
Lower factors are more typical for rural freeways or off-peak
conditions. Higher factors are typical of urban and suburban peakhour conditions.
.
Webster’s Design Method
 It has been found from studies that the average delay and the
overall delay to the vehicles at a signalized intersection very with the
signal cycle length.
 The average delay per vehicle is high when the cycle length is very
less, as sizable proportion of vehicles may not get cleared during the
first cycle and may spill over to subsequent cycles.
As the signal cycle time is increased, the average delay per vehicle
decreases up to a certain minimum value and thereafter the delay
starts increasing, indicating that there is an “optimum signal cycle
time” corresponding to least overall delay. 4.
The optimum cycle time depends on the geometric details of the
intersection and the volume of traffic approaching the intersection
from all the approach roads during the design hour.5. The field work
consists of determining the following two sets of values on each
approach road near the intersection: The normal flow, q on each
approach during the design hour and the „saturation flow‟, S per
unit time. The normal flow values, q1 and q2 on roads 1 and 2 are
determined from field studies conducted during the design hour or
the traffic during peak 15 – minute‟s period. 7
. The saturation flow of vehicles is determined from careful field
studies by noting the number of vehicles in the stream of compact
flow during the green phases and the corresponding time intervals
precisely. In the absence of data the approximate value of saturation
flow is estimated assuming 160 PCU per 0.3 metre width of the
approach
MULTIPLIED BY CORRESPONDING PCU FACTORS
MULTIPLIED BY CORRESPONDING PCU FACTORS
MULTIPLIED BY CORRESPONDING PCU FACTORS
MULTIPLIED BY CORRESPONDING PCU FACTORS
MULTIPLIED BY CORRESPONDING PCU FACTORS
MULTIPLIED BY CORRESPONDING PCU FACTORS
MULTIPLIED BY CORRESPONDING PCU FACTORS
MULTIPLIED BY CORRESPONDING PCU FACTORS
These are the PCU factors
For 9:00 AM-10:00 AM
Green signal time for traffic moving from college=60 seconds
Pedestrian crossing time=15 seconds
Green signal time for traffic moving from Khairatabad
And SR Nagar=85 seconds
All red signal time =15 seconds
Green signal time for traffic moving from Begumpet=30 seconds
For 6:00 PM-7:00 PM
Green signal time for traffic moving from college=55 seconds
Pedestrian crossing time=10 seconds
Green signal time for traffic moving from Khairatabad
And SR Nagar=90 seconds
All red signal time =10 seconds
Green signal time for traffic moving from Begumpet=40 seconds
TRAFFIC MOVING FROM COLLEGE TOWARDS
KHAIRATABAD, BEGUMPET AND SR NAGAR
qa = 400.7 PCU/hr
qb= 362.1 PCU/hr
qc= 503 PCU/hr
Sa= total number of vehicles *cycle length
total number of vehicles moving in green time
=736*145 =1778
60
Sb= total number of vehicles *cycle length
total number of vehicles moving in green time
=418*145
1010.16
60
Sc= total number of vehicles *cycle length
total number of vehicles moving in green time
=965*145 = 2340
60
n=4
R = 15 seconds
ya = qa = 400.7 =0.224
sa
1778
yb = qb = 362.1 =0.358
sb
875
yc = qc = 503
sc
=0.214
2340
Y = ya + yb + yc = 0.796
L=2n + R = 23 seconds
C0= 1.5*L+5 193.6
1-Y
Ga=Ya (C0-L) = 47.93
Y
Gb=Yb (C0-L) = 76.6
Gc=Yc (C0-L) 45.89
Y
Provide 2 seconds amber time
=47.93+76.6+45.89+2+2+15
=189.92 seconds
TRAFFIC MOVING FROM KHAIRATABAD TOWARDS
COLLEGE AND SR NAGAR
qa =354 PCU/hr
qb=651.1 PCU/hr
Sa= total number of vehicles *cycle length
total number of vehicles moving in green time
=(1277*145)/85 = 2178.4
Sb= total number of vehicles *cycle length
total number of vehicles moving in green time
=(685*145)/85 = 1168.5
n=4
R = 15 seconds
ya = qa = 354.1/1168.5 = 0.302
sa
yb = qb =651.1/2178.4 =0.298
sb
Y = ya + yb = 0.6
L=2n + R = 23 seconds
C0= 1.5*L+5 = 68.75
1-Y
Ga=Ya (C0-L) = 27.0365
Y
Gb=Yb (C0-L) =26.66
Y
Provide 2 seconds amber time
26.66+27.0365+15+2+2
=72.6965 seconds
TRAFFIC MOVING FROM SR NAGAR TOWARDS
KHAIRATABAD
qa =1519.4 PCU/hr
Sa= total number of vehicles *cycle length
total number of vehicles moving in green time
= (1955*145)/85
=3335
n=4
R = 15 seconds
ya = qa = 1519.4/3335 = 0.455
sa
Y = ya =0.455
L=2n + R = 23 seconds
C0= 1.5*L+5 = 72.47
1-Y
Ga=Ya (C0-L) = 49.47
Y
Provide 2 seconds amber time
49.74 + 15 + 2 + 2=68.75 seconds
TRAFFIC MOVING FROM COLLEGE TOWARDS
KHAIRATABAD, BEGUMPET AND SR NAGAR
qa =310.1
qb=192.8
Sa= total number of vehicles *cycle length
total number of vehicles moving in green time
=(491*145)/30 = 2373.1
Sb= total number of vehicles *cycle length
total number of vehicles moving in green time
=(170*145)/30 = 831.66
n=4
R = 23
ya = qa =0.13
sa
yb = qb =0.234
sb
Y = ya + yb = 0.364
L=2n + R = 23seconds
C0= 1.5*L+5 = 73.9
1-Y
Ga=Ya (C0-L) =19.24
Y
Gb=Yb (C0-L) =34.6
Y
Provide 2 seconds amber time
34.6 + 19.24 + 15+ 2 + 2
= 72.84 seconds
TRAFFIC MOVING FROM COLLEGE TOWARDS
KHAIRATABAD, BEGUMPET AND SR NAGAR
qa = 514.4 PCU/hr
qb= 574.4 PCU/hr
qc= 571.7 PCU/hr
Sa= total number of vehicles *cycle length
total number of vehicles moving in green time
=(744*205)/55 = 2773
Sb= total number of vehicles *cycle length
total number of vehicles moving in green time
=(602*205)/55 = 2343.8
Sc= total number of vehicles *cycle length
total number of vehicles moving in green time
=(1122*205)/55 = 4182
n=4
R = 20 seconds
ya = qa = 0.185
sa
yb = qb =0.245
sb
yc = qc =0.136
sc
Y = ya + yb + yc = 0.566
L=2n + R =28 seconds
C0= 1.5*L+5 = 108.3 seconds
1-Y
Ga=Ya (C0-L) = 26.1778 seconds
Y
Gb=Yb (C0-L) =34.6896 seconds
Y
Gc=Yc (C0-L) = 19.272 seconds
Y
Provide 2 seconds amber time
26.1778+34.6896+19.272+20+2+2
=104.1394 seconds
TRAFFIC MOVING FROM KHAIRTABAD TOWARDS
SR NAGAR AND COLLEGE
qa = 819.4 PCU/hr
qb= 460.5 PCU/hr
Sa= total number of vehicles *cycle length
total number of vehicle moving in green time
=(1498*205)/90 = 3412.11
Sb= total number of vehicles *cycle length
total number of vehicles moving in green time
=(724*205)/90 = 1649.11
n=4
R = 20 seconds
ya = qa = 0.24
sa
yb = qb =0.279
sc
Y = ya + yb =0.519
L=2n + R = 28 seconds
C0= 1.5*L+5 = 97.7 seconds
1-Y
Ga=Ya (C0-L) = 32.06 seconds
Y
Gb=Yb (C0-L) =36.941 seconds
Y
Provide 2 seconds amber time
=32.06+36.941+20+2+2
=86.93 seconds
TRAFFIC MOVING FROM SR NAGAR TOWARDS
KHAIRATABAD
qa = 1043.2 PCU/hr
Sa= total number of vehicles *cycle length
total number of vehicles moving in green time
=(1542*205)/90
=3512.33
n =4
R = 20 seconds
ya = qa =0.279
sa
Y = ya = 0.279
L=2n + R = 28 seconds
C0= 1.5*L+5 = 66.86 seconds
1-Y
Ga=Ya (C0-L) = 46.86 seconds. ,
Y
Provide 2 seconds amber time
=46.86+2+2
=50.86 seconds
TRAFFIC MOVING FROM BEGUMPET TOWARDS SR
NAGAR AND COLLEGE
qa = 448.5 PCU/hr
qb= 275.6 PCU/hr
Sa= total number of vehicles *cycle length
total number of vehicles moving in green time
=(640*205)/40
=3280
Sb= total number of vehicles *cycle length
total number of vehicles moving in green time
=(277*205)/40
=1419.625
n=4
R = 20 seconds
ya = qa =0.136
sa
yb = qb =0.194
sb
Y = ya + yb = 0.33 seconds
L=2n + R = 28 seconds
C0= 1.5*L+5 =70.15 seconds
1-Y
Ga=Ya (C0-L) =50.15 seconds
Y
Gb=Yb (C0-L) =29.478 seconds
Y
Provide 2 seconds amber time
20.66+29.74+2+2
=74.138 seconds