Chapter 5 — Traffic Engineering Toolbox
Overview
It is often beneficial to have a basic understanding of when certain types of intersection
control or geometric modifications are considered by traffic engineers. In addition to a
variety of guidelines, methodologies and rules of thumb, traffic engineers utilize several
software tools to model a variety of situations in great detail. This detailed level of analysis is
not always feasible or cost effective when making bigger picture planning level decisions. To
assist County staff, several sections are presented that summarize information typically used
by traffic engineers to determine the type of intersection control that should be considered
(multiway stop, roundabout, or traffic signal), when exclusive turn lane may be beneficial,
and what basic roadway sections should be considered based on anticipated traffic demand.
A brief description of some commonly used traffic analysis software tools is also included.
It is important to note that this section provides relatively general information and there are
situations where the advice of or further analysis by a traffic engineer is justified.
Multiway Stop Warrants
Chapter 2B of the current version (2003 Edition
Revision 1) of the Manual of Uniform Traffic Control
Devices (MUTCD), published by the Federal
Highway Administration (FHWA), contains
warrants that should be used to help determine if
multiway stop control should be considered as a
method of intersection control. The MUTCD is
available online at http://mutcd.fhwa.dot.gov.
Multiway stop control can be a useful safety
measure but should be justified or compliance may
diminish which could result in potential safety
concerns.
An engineering study should be the basis of a decision to install multiway stop control. The
MUTCD (pg. 2B-8) states that the following criteria should be considered in the engineering
study for a multiway stop controlled intersection:
A. Where traffic control signals are justified, the multiway stop is an interim measure that can be
installed quickly to control traffic while arrangements are being made for the installation of the
traffic control signal.
B. A crash problem, as indicated by 5 or more reported crashes in a 12-month period that are
susceptible to correction by a multiway stop installation. Such crashes include right- and left-turn
collisions as well as right-angle collisions.
C. Minimum volumes:
1. The vehicular volume entering the intersection from the major street approaches (total of both
approaches) averages at least 300 vehicles per hour for any 8 hours of an average day, and
2. The combined vehicular, pedestrian, and bicycle volume entering the intersection from the minor
street approaches (total of both approaches) averages at least 200 units per hour for the same 8
hours, with an average delay to minor-street vehicular traffic of at least 30 seconds per vehicle during
the highest hour, but
3. If the 85th-percentile approach speed of the major-street traffic exceeds 65 km/h or exceeds 40
mph, the minimum vehicular volume warrants are 70 percent of the above values.
D. Where no single criterion is satisfied, but where Criteria B, C.1, and C.2 are all satisfied to 80
percent of the minimum values. Criterion C.3 is excluded from this condition.
In addition to the previously mentioned guidelines, several conditions should be considered
in the engineering study. These conditions include control of left-turn conflicts,
vehicle/pedestrian conflicts, limited sight distance, and impacts on mobility. If a multiway
stop controlled intersection is being considered, Section 2B.07 of the MUTCD should be
reviewed because only a summary of that section is included in this report.
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Roundabout Guidelines
Traffic Signal Warrants
Currently, no warrants are used to justify a roundabout. However, general policy suggests a
roundabout can be considered if the criteria required to install an all-way stop are satisfied.
FHWA has published a guide that includes general principles of roundabout design
(Roundabouts: An Informational Guide, available from the Turner-Fairbank Highway Research
Center website at www.tfhrc.gov). Figure 5.1 shows the relationship between the maximum
entry volume for a roundabout approach and the circulatory flow within a single-lane
roundabout. The figure shows two lines – one for an urban compact roundabout and one
for an urban or rural single lane roundabout. The urban compact roundabout typically will
not be applicable on county roadways due to its tighter center radius and entry geometrics. If
the plotted point falls above the dashed line, the capacity of a single-lane roundabout has
been exceeded and a two-lane roundabout or other method of intersection control should be
considered. When completing this analysis the capacity of each entry point needs to be
considered. If a more detailed review is required, it is recommended that a traffic engineer
complete a detailed traffic analysis using a software package such as RODEL.
The Manual of Uniform Traffic Control Devices (MUTCD) also contains guidelines to help decide
if the installation of a traffic signal should be considered. The MUTCD requires that an
engineering study be performed to determine whether the installation of a traffic control
signal is justified at a particular location. The MUTCD contains the following warrants that
if satisfied may be one factor that justifies the installation of the traffic signal:
Figure 5.1 – Maximum Entry Volume and Circulatory Flow
(Source: Roundabouts: An Informational Guide)
Warrant 1 – Eight-Hour Vehicular Volume
Warrant 5 – School Crossing
Warrant 2 – Four-Hour Vehicular Volume
Warrant 6 – Coordinated Signal System
Warrant 3 – Peak Hour
Warrant 7 – Crash Experience
Warrant 4 – Pedestrian Volume
Warrant 8 – Roadway Network
A need for a traffic signal should be considered if one of the warrants is satisfied. A traffic
control signal should not be installed unless an engineering study indicates that installing a
traffic control signal will improve the overall safety and/or operation of the intersection.
Many of the warrants are only used in unique situations that are applicable on occasion.
These situations typically are the peak hour, pedestrian volume, school crossing, coordinated
signal system, and roadway network warrants. A brief description of these warrants is
included, but the MUTCD should be consulted for more information.
The peak hour warrant is only intended to be used in unique situations with a very large
traffic generator, such as a manufacturing plant or office complex, that requires a signal due
to such a large number of vehicles entering or leaving the site at the same time. While this
warrant was used routinely to justify traffic signals in the past, it is not typically allowed by
most jurisdictions unless a unique traffic generator exists in the area. The pedestrian volume
warrant and school crossing warrant are related to the number of pedestrians or students
that cross at a particular intersection and considers whether they have an adequate number
of gaps in traffic to cross the street. These are applied to locations with heavy pedestrian
movements and cross traffic or along walking routes to school that cross a major roadway.
The coordinated signal system warrant allows a signal to be installed where it may otherwise
be unjustified if it helps maintain groups of vehicles that are traveling down a coordinated
signalized corridor. The roadway network warrant is similar in that a traffic signal may be
justified at an intersection of two or more major roadways if the signal would help maintain
organized traffic flow.
The most commonly applied traffic signal warrants are the eight-hour vehicular volume,
four-hour vehicular volume and the crash experience warrants. The following sections
describe these warrants in more detail.
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Warrant 1: Eight-Hour Vehicular Volume
The MUTCD describes two conditions under the eight-hour vehicular volume warrant.
Condition A is used when the volume on the major roadway is the reason for signal
consideration, and Condition B is used when the volumes on the side street are the reason
for signal consideration. If either Condition A, B, or a combination of the two are satisfied,
then Warrant 1 is satisfied. The following are excerpts from the MUTCD:
Standard:
The need for a traffic control signal shall be considered if an engineering study finds that one of the
following conditions exist for each of any 8 hours of an average day:
Standard:
The need for a traffic control signal shall be considered if an engineering study finds that both of the
following conditions exist for each of any 8 hours of an average day:
A. The vehicles per hour given in both of the 80 percent columns of Condition A in
Table 5.1 exist on the major-street and the higher-volume minor-street approaches,
respectively, to the intersection; and
Table 5.1 – Warrant 1, Eight-Hour Vehicular Warrant (Source: MUTCD)
B. The vehicles per hour given in both of
the 80 percent columns of Condition B
in Table 5.1 exist on the major-street
and the higher-volume minor-street
approaches, respectively, to the
intersection.
A. The vehicles per hour given in both of the 100 percent columns of Condition A in
Table 5.1 exist on the major-street and the higher-volume minor-street approaches,
respectively, to the intersection; or
B. The vehicles per hour given in both of the 100 percent columns of Condition B in
Table 5.1 exist on the major-street and the higher-volume minor-street approaches,
respectively, to the intersection.
In applying each condition the major-street and minor-street volumes shall be for the same 8 hours.
On the minor street, the higher volume shall not be required to be on the same approach during each
of these 8 hours.
Option:
If the posted or statutory speed limit or the 85th-percentile speed on the major street exceeds 70
km/h or exceeds 40 mph, or if the intersection lies within the built-up area of an isolated
community having a population of less than 10,000, the traffic volumes in the 70 percent columns
in Table 5.1 may be used in place of the 100 percent columns.
Guidance:
The combination of Conditions A and B is intended for application at locations where Condition A
is not satisfied and Condition B is not satisfied and should be applied only after an adequate trial of
other alternatives that could cause less delay and inconvenience to traffic has failed to solve the traffic
problems.
These major-street and minor-street volumes shall
be for the same 8 hours for each condition;
however, the 8 hours satisfied in Condition A
shall not be required to be the same 8 hours
satisfied in Condition B. On the minor street,
the higher volume shall not be required to be on
the same approach during each of the 8 hours.
Option:
If the posted or statutory speed limit or the 85thpercentile speed on the major street exceeds 70
km/h or exceeds 40 mph, or if the intersection
lies within the built-up area of an isolated
community having a population of less than
10,000, the traffic volumes in the 56 percent
columns in Table 5.1 may be used in place of
the 80 percent columns.
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Warrant 2: Four-Hour Vehicular Volume
Warrant 7: Crash Experience
The four-hour vehicular volume warrant is primarily design to apply where volumes of
intersecting traffic is the primary reason a signal may be justified. The following are excerpts
from the MUTCD:
The crash experience warrant is applicable to locations where
accidents are the primary reason why a traffic signal is being
considered. The following are excerpts from the MUTCD:
Support:
Support:
The Four-Hour Vehicular Volume signal warrant conditions are intended to be applied where the
volume of intersecting traffic is the principal reason to consider installing a traffic control signal.
The Crash Experience signal warrant conditions are intended for
application where the severity and frequency of crashes are the
principal reasons to consider installing a traffic control signal.
Standard:
The need for a traffic control signal shall be considered if an engineering study finds that, for each of
any 4 hours of an average day, the plotted points representing the vehicles per hour on the major
street (total of both approaches) and the corresponding vehicles per hour on the higher-volume minorstreet approach (one direction only) all fall above the applicable curve in Figure 5.2 for the existing
combination of approach lanes. On the minor street, the higher volume shall not be required to be on
the same approach during each of these 4 hours.
Option:
If the posted or
statutory speed
limit or the 85thpercentile speed on
the major street
exceeds 70 km/h
or exceeds 40 mph
or if the
intersection lies
within the built-up
area of an isolated
community having
a population of less
than 10,000,
Figure 5.3 may
be used in place of
Figure 5.2.
Figure 5.2 – Warrant 2, Four-Hour Vehicular Volume (Source: MUTCD)
Figure 5.3 – Warrant 2, Four-Hour Vehicular Volume (70% Factor)
(Source: MUTCD)
(Community less than 10,000 population or above 40 mph on major street)
Standard:
The need for a traffic control signal shall be considered if an
engineering study finds that all of the following criteria are met:
A. Adequate trial of alternatives with satisfactory
observance and enforcement has failed to reduce the
crash frequency; and
B. Five or more reported crashes, of types susceptible to
correction by a traffic control signal, have occurred
within a 12-month period, each crash involving personal
injury or property damage apparently exceeding the
applicable requirements for a reportable crash; and
C. For each of any 8 hours of an average day, the vehicles per hour (vph) given in both of the
80 percent columns of Condition A in Table 5.1 (see Section 4C.02), or the vph in both
of the 80 percent columns of Condition B in Table 5.1 exists on the major-street and the
higher-volume minor-street approach, respectively, to the intersection, or the volume of
pedestrian traffic is not less than 80 percent of the requirements specified in the Pedestrian
Volume warrant. These major street and minor-street volumes shall be for the same 8
hours. On the minor street, the higher volume shall not be required to be on the same
approach during each of the 8 hours.
Option:
If the posted or statutory speed limit or the 85th-percentile speed on the major street exceeds 70
km/h or exceeds 40 mph, or if the intersection lies within the built-up area of an isolated
community having a population of less than 10,000, the traffic volumes in the 56 percent columns
in Table 5.1 may be used in place of the 80 percent columns.
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Exclusive Turn Lane Considerations
Exclusive turn lanes provide two primary functions – they increase safety by removing
turning vehicles that are decelerating from the through traffic lane and increase the capacity
and improve operations at an intersection. The guidelines for when turn lanes should be
considered vary across the country. A general rule used by WisDOT is a left turn lane should
be considered if in the construction year the average annual daily traffic (AADT) volume on
the main road exceeds 7,000 vehicles per day (vpd) and the side road volume exceeds 1,000
vpd. WisDOT also has a left turn lane warrant in procedure 11-25-5 of the Facilities
Development Manual that applies to facilities with relatively high operating speeds (60 mph).
The storage length can be estimated using a variety of formulas or by completing a traffic
analysis that estimates the anticipated queue length (typically the 95th percentile queue
length). It is particularly important that additional analysis be completed at signalized and
stop controlled approaches. A rule of thumb often used to estimate storage is the storage
length in feet equals the number of vehicles turning during the peak hour (that is, if 200
vehicles turn then 200 feet is required for storage). Regardless of the results of the storage
value, the turn lane should be at full width for a minimum length of 50 feet with 100 feet
preferred.
The Olmsted County (located in southeastern Minnesota) Access Management Policy
recommends that right turn lanes be considered at all public street intersections where
speeds are greater than 40 mph and existing or projected mainline volumes are greater than
1,500 vpd. It also recommends right turn lanes at access points that will serve more than 10
residential units or a business that generates more than 75 turns per day. In Colorado,
exclusive turn lanes typically are required on roadways with 10 vehicles turning left and 25
vehicles turning right during the peak hour.
The maneuver distances documented in the Facility Development Manual is based on the speed
of the roadway:
Based upon a review of several turn lane policies reviewed, left turn lanes are recommended
for consideration when more than 25 vehicles turn left during the peak hour. Right turn
lanes should be considered when more than 50 vehicles turn right during the peak hour.
Turn lanes also should be considered when accident data indicates a significant number of
accidents have occurred that could be remedied by the installation of an exclusive turn lane
or when geometric conditions justify their installation (turn near the crest of a vertical curve,
etc.).
The total length of a turn lane, from the beginning of the taper to the corner of the
intersection, is composed of two distances – the maneuver distance (the distance vehicles
move from the through lane to the turn lane) and the storage length (the distance vehicles
stack to wait for gaps in traffic or for the traffic signal to give a green indication). Right turn
lanes on the primary roadway that do not have to stop generally are sized to accommodate
only the maneuver distance. The typical taper rate used by WisDOT is 12.5 to 1, which
translates to a 150’ taper for a turn lane that is 12 feet wide. The length of the turn lane at
full width is equal to the maneuver distance plus the storage distance (if applicable) minus
the taper length.
Speed (mph)
Maneuver Distance (ft)
30
160
40
275
50
425
For example, a left lane is being planned for a left turning volume of 25 vehicles on a 30
mph roadway. The storage required is 30 feet using the one foot per vehicle rule of thumb.
The maneuver distance is 160 feet. The overall turn lane length, then, is 190 feet (160 feet
plus 30 feet). Removing the 150-foot taper leaves a full width segment of only 40 feet.
Because this length is below the desired length, the length should be increased to 100 feet.
As a result, the left turn lane would have a 150-foot taper and a 100-foot full width section.
A right turn lane on an approach that does not have to stop operates at a speed of 50 mph.
Since a right turn lane does not require storage if traffic does not have to stop, the total
length is equal to the maneuver distance which is 425 feet. The length of the full width
section is equal to 425 minus 150 or 275 feet. So, a turn lane with a 150-foot taper and 275foot full length section is recommended.
Length of Turn Lane at Full Width = (Maneuver Distance + Storage Distance) - Taper Length
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5-5
Capacities of Common Roadway Sections
Traffic Analysis Software Tools
A variety of general planning
tables provide guidance regarding
the capacities of typical roadway
sections used across the country.
These tables typically are derived
using methodologies from the
Highway Capacity Manual.
Examples from the upper
Midwest found in City planning
documents are provided below.
It is important to note that the
capacity can vary greatly and these
examples should only be used for
general planning purposes.
Numerous software tools are used to model traffic operations for a variety of intersection
configurations and facility types such as freeway (basic segments, merge, diverge, and weave
conditions), two-way or multiway unsignalized intersections, signalized intersections,
roundabouts, etc. A basic summary of the most commonly used tools is presented below.
This information can be used to help County staff understand the tools used when they
review traffic studies.
Table 5.2 – Typical Roadway Capacities
Cross-Section
Two-lane Urban
Three-lane Urban
Four-lane Undivided Arterial
Four-lane Divided Arterial
Six-lane Divided Arterial
Four-lane Expressway
Four-lane Unmetered Freeway
General Maximum
Two-Way ADT*
8,000 to 10,000
14,000 to 18,000
15,000 to 25,000
30,000 to 41,000
45,000 to 60,000
35,000 to 60,000
65,000 to 90,000
* Capacity can vary greatly depending on access control, cross-street
volumes, and peaking characteristics. These values reflect potential capacity
and not desirable range of operation.
Table 5.2 is included in the 2000 Comprehensive Plan for the City of Edina, Minnesota
shows typical general maximum two-way average daily traffic.
Table 5.3 from the City of Middleton’s (located in Wisconsin) Traffic Impact Analysis
Guidelines shows approximate street system planning capacities.
Table 5.3 – Approximate Street System Planning Capacities
Cross-Section
Level of Service Volume (vpd)*
2-Lane Undivided without Turn Lanes
2-Lane Undivided with Turn Lanes
4-Lane Undivided without Turn Lanes
4-Lane Undivided with Turn Lanes
4-Lane Divided with Turn Lanes
5-Lane with Two-Way Left Turn Lanes
6-Lane Divided with Turn Lanes
LOS C
13,000
15,000
17,000
21,000
25,000
30,000
35,000
LOS D
15,000
17,000
19,500
24,000
29,000
35,000
40,500
HCS
HCS (Highway Capacity Software) implements the methodologies documented in the
Highway Capacity Manual. The Highway Capacity Manual, published by the Transportation
Research Board, includes procedures used extensively across the United States to assess
traffic operations for a variety of facility and intersection configurations. The HCS includes
modules to analyze capacity and estimate level of service (LOS) for a variety of scenarios
that include but are not limited to signalized intersections, unsignalized intersections,
arterials, multilane highways, two-lane highways, and freeways.
Synchro and SimTraffic
Synchro, by Trafficware, is popular among traffic engineers across the country. This
software incorporates the principles of the Highway Capacity Manual. It is used to estimate
the level of service and analyze the capacity of signalized, unsignalized, and arterial
components of the roadway network. Synchro also is a traffic signal timing tool often used
to develop timing plans. Several signal timing optimization features assist the user in
determining the optimum cycle length, signal splits (green times per movement), and signal
offsets (coordination parameters). Synchro is a macroscopic model, which means a series of
equations are used to estimate traffic conditions. Macroscopic models have limited ability to
estimate the impact of adjacent intersections and situations when traffic volumes approach
or exceed capacity. Macroscopic models routinely are used because they are relatively cost
effective and usually provide the level of detail required when compared to more detailed
modeling tools.
* Capacity can vary greatly depending on access control, cross-street volumes, and peaking
characteristics. All street cross-sections include on-street parking on both sides. The table is based on
former WisDOT FDM planning values which have been supplemented by iterative HCS
calculations.
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SimTraffic, also by Trafficware, allows the modeler to take the Synchro model input and
model conditions microscopically. Microscopic models model each vehicle individually and
the output is based on data collected as each vehicle progresses through the model.
SimTraffic allows the engineer to more accurately assess impacts of an adjacent intersection
on operations and situations when the roadway network or intersection is operating near or
over capacity. Microscopic models generally do not have the ability to optimize signal
timings or other parameters but rather model conditions that are input directly. SimTraffic
also has an animation feature that allows the user to watch traffic progress through the
model. A microscopic model is stochastic (involves a random variable), so it is important
that the modeler complete several model runs and average the results. Typically, five runs are
completed for each scenario modeled.
RODEL
RODEL is an empirically based model developed in the United Kingdom that is used to
evaluate whether a roundabout is feasible and assess the impacts of geometric design
features (radius, entry flare, entrance width, etc.) on traffic operations. This tool is relatively
straight forward to use and has been accepted as the required tool in many jurisdictions
across the country. The WisDOT Facilities Development Manual requires that RODEL be used
in roundabout analyses completed for WisDOT. The user must note that RODEL only
evaluates isolated intersections, so if roundabouts are closely spaced or a proposed
roundabout is located close to a signalized intersection, another modeling tool should be
used to properly evaluate the impacts of the adjacent intersections.
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