This case study presents information on a decision that will
be made by the Idaho Transportation Department regarding the operation of an
intersection on U.S. Highway 95 within the City of Moscow, Idaho. The
intersection is currently stop-sign controlled but consideration is being given
to installing a traffic signal at the intersection. Click here to
see what kinds of problems will be considered as part of this case study.
The case study includes six problems, each one illustrating
some aspect of this decision:
Problem 1 analyzes the intersection under both stop sign and signal control;
Problem 2 addresses the effects of
adjacent intersections as well as progression considerations;
Problem 3 illustrates how to
evaluate the intersection under oversaturated conditions, and also how to
take account of a significant change in vehicle mix;
Problem 4 addresses the issue of
Problem 5 suggests how to deal with
an adjacent road segment that is neither an arterial nor a two-lane highway;
Problem 6 shows how a
planning-level analysis might be conducted.
The purpose of each of the problems is to
show how various traffic analysis tools that are contained in the Highway
Capacity Manual can be applied to assist traffic analysts, engineers, planners
and decision-makers in making sound investment decisions regarding the
signalization decision. There are some situations where this amount of analysis
would not be necessary in order to make an informed decision, but the issues
presented herein should always be considered so as to assure the final decision
is consistent with system performance objectives.
The problems in this case study focus on the chapters of the
HCM that deal with interrupted flow facilities, either signalized or
unsignalized intersections. After studying this material, you should be
Analyze the operation of signalized intersections,
unsignalized intersections, and urban arterials using the HCM.
Understand what input data are required and the
assumptions that are commonly made regarding default values for the HCM
procedures for these facilities.
Know the appropriate kinds of analysis that should be
undertaken for both existing facilities as well as future facilities or conditions, including the scope of a facility that should be included
in an analysis.
Understand the limitations of the HCM procedures and
when it is appropriate to use other models or computational tools.
Know how to reasonably interpret the results from an
HCM analysis and how these results can be used to support a particular
decision regarding a change to a transportation system.
The intersection of U.S. 95 with Styner Avenue-Lauder
Avenue is currently stop-controlled, and is the focus of a controversy over
whether a signal ought to be installed.
To understand the context in which our analyses will be
conducted, it is important to note that U.S. 95 is the primary north-south state highway in Idaho,
carrying between 15,000 and 23,000 vehicles per day through the city of
Moscow, a community of 21,000 persons located in north Idaho. The city is home to the University of Idaho, the state's land grant
university with a student population of nearly 11,000.
Traffic on this section of
U.S. 95 has been increasing at the rate of nearly
two percent per year over the past decade.As it enters the city from the south, U.S. 95 transitions from
a two-lane highway to an urban arterial with both signal and stop-sign
control. Much of the land use to the south is farming, with some
industrial, commercial, and residential development encroaching into these
less developed sections of the area.
The area to the east of
U.S. 95 in the southern part of the
city is primarily residential, particularly along Styner Avenue and
Palouse River Drive. Click the link
to Exhibit 1-1 on the left to see this section of the city's highway system.
In the southern part of the city,
U.S. 95 transitions from a
two-lane highway to a four-lane arterial. Two of the intersections
(State Highway 8 and Sweet Avenue) are controlled by traffic signals while
three of the intersections (Taylor Avenue, Styner Avenue, and Palouse River
Drive) are controlled by stop signs.There
are also a number of driveway access points along the arterial. Click on Exhibit
1-2 to see an overview sketch of the U.S. 95 corridor and Exhibit 1-3 to see
an aerial view of U.S. 95.
The intersections of
95/State Highway 8 (see Exhibit 1-4) and
U.S. 95/Sweet Avenue
(see Exhibit 1-5) are signal controlled,
operating fully-actuated, with no coordination between them.The three unsignalized intersections are located at Taylor Avenue/U.S. 95, Styner-Lauder Avenue/U.S.
95 (see Exhibit 1-6) and Palouse River Drive/U.S. 95.
Click the links to the
left to view aerial photographs of these intersections.
Exhibit 1-1. Sketch
overview of Moscow highway system
The U.S. 95 corridor in the city of Moscow is shown in the
figure below. The corridor runs from State Highway 8 on the north to Palouse River Drive on the south. Two signalized intersections and two
two-way stop-controlled intersections are located on this section of the U.S. 95
corridor. The corridor extends south of the city
and transitions into a multilane highway and then a two lane rural
State Highway 8 is an east-west highway through the
city. In the section shown below, it is two lanes, one in each
direction. Two signalized intersections and two TWSC intersections are
located on this section of the highway.
Note: You can see aerial photographs of several of the
intersections by moving the mouse over the intersections and clicking on them.
Exhibit 1-2. Sketch overview
of U.S. 95 corridor in Moscow
This sketch shows the geometrical detail of the
corridor between State Highway 8 and Palouse River Drive.
The intersection of U.S. 95 and State Highway 8 is a
signalized intersection. The intersection has three approach legs.
The intersection of U.S. 95 and Sweet Avenue is a signalized
T-intersection. Sweet Avenue serves as the south entry to the University
The intersection of U.S. 95 and Styner-Lauder Avenue
is a TWSC intersection. Styner Avenue serves a growing residential area in
the eastern section of the city. Lauder Avenue serves as a minor entrance
to the University of Idaho and access to student and faculty residences.
The intersection of U.S. 95 and Palouse River Drive is a TWSC
intersection. The eastern approach of Palouse River Drive is scheduled for
major improvements as it may become the major service and access road for the
eastern section of the city.
Exhibit 1-3. Aerial photograph of
U.S. 95 corridor in Moscow
The aerial photograph shows the
U.S. 95 corridor as it runs
north-south in the City of Moscow from the State Highway 8 intersection on the
north to the Styner Avenue/Lauder Avenue intersection on the south.
Move the cursor over the photograph to zoom in on the
intersections of State Highway 8, Sweet Avenue, and Styner Avenue/Lauder Avenue.
Exhibit 1-4. Aerial photograph of
U.S. 95/State Highway 8
The intersection of U.S. 95 and State Highway 8 is located at
the southern end of Moscow's central business district. U.S. 95 continues to
the north as the northern portion of the downtown's one-way couplet. The
southern portion of the couplet is the eastbound approach to this
intersection. The eastbound approach serves traffic coming from Moscow's
east side and smaller communities such as Troy and Deary to the east.
Exhibit 1-5. Aerial photograph of
U.S. 95/Sweet Ave
The intersection of U.S. 95 and Sweet Avenue is a signalized
T intersection. The eastbound Sweet Avenue approach serves as the south
entrance to the University of Idaho. It is a four lane boulevard with a
median strip. U.S. 95 is has turning lanes from the north and south to serve
university traffic. The intersection actually functions as a four leg
intersection with access provided to a business located on the westbound
approach to the intersection but traffic volumes on this approach are so minimal
that they can be ignored.
Exhibit 1-6. Aerial Photograph of
the Intersection of U.S. 95 with Styner Ave/Lauder Ave
The intersection of U.S. 95/Styner Avenue/Lauder Avenue is a
four leg TWSC intersection. The westbound approach, Styner Avenue, serves
a growing residential area in the eastern section of the city. Lauder
Avenue serves as a minor entrance to the University of Idaho and access to
student and faculty residences. A gas station/mini-mart is located on the
northwest quadrant of the intersection. Other auto-oriented businesses are
located on the other quadrants.
Note: You can see other views of the intersection approach by
moving your mouse to the approach and clicking on the approach.
have asserted that increasing traffic volumes at the intersection of Styner
Avenue, Lauder Avenue, and U.S. 95 are increasing delays for and reducing
the safety of motorists traveling through the intersection.Traffic on Styner and Lauder, the minor streets of this two-way
stop-controlled intersection, is controlled by stop signs.Limited sight distance for drivers on Styner and Lauder contributes to
the safety problem.As a result of
these citizen complaints, the city has requested that the Idaho
Transportation Department signalize the intersection.
Discussion: Consider the situation
described above and review the sketches and aerial photographs showing the
street system. What issues do you think should be considered by the state's
traffic engineer in resolving this problem? After you have listed these
issues, continue to the next page to learn how we can apply the methods of
the Highway Capacity Manual to address these issues and provide assistance
to decision makers from the state and the city on whether or not this
intersection should be signalized.
As we begin this case study, we will
consider several issues, including the scope of the analysis that we will
undertake, the stakeholders and the issues that are important to them, the
goal of the case study, the performance measures that we will use to support
this decision-making process, the analyses that we will perform to generate
our performance measures, the tools that we will use, the data that is
required, and some of the technical issues with respect to the Highway
Capacity Manual that are included in each problem. Click here to
see what kinds of problems will be considered as part of this case study.
Scope of the Analysis Our major focus is the intersection of
U.S. 95 with Styner Avenue and
Lauder Avenue. But we will see as our discussion proceeds that it is
important to consider not just this intersection but some of the surrounding
highway system as well. For now, we'll define the segment of
most interest to us as the U.S. 95 corridor from State Highway 8 on the north to Palouse River Drive on the south.
We will also consider travel patterns that normally occur
during a typical weekday, while the University of Idaho is in session. Traffic volumes vary throughout this typical weekday, with peak periods
occurring during the morning and afternoon, with a secondary peak during the
midday. There are also mini-peak periods during class change times, but
these variations are not significant enough to consider for our analysis
here. We'll see later in the analysis, however, that there are other
time periods, such as during special university events, that traffic volumes
increase significantly and have a bearing on the results that we need to
While many people are concerned about the operation of the
intersection today, it is also important to consider how the intersection is
likely to operate in the future. The investment in signal controllers
and related equipment is sizable, and its feasibility must be justified by
looking at both conditions present today and those likely to exist in the
future. For this reason, we will also consider future traffic volume
projections for the U.S. 95 corridor.
Stakeholders The Idaho Transportation Department has primary authority for the
operation of U.S. 95. It does all of the planning, design, and maintenance of
this highway. However, it works closely with the staff from the City
of Moscow so that, as much as possible, decisions regarding the state
highway facilities within city limits are made jointly between the city and
the state. As we develop the results from our analyses, the major
audience, then, will be the engineering staffs from these two agencies.
In addition, residents of the city will need to understand
the rationale for any final decision that is made regarding the intersection
and whether a signal should be installed or not. This points up an
important issue in any technical analysis. While any analysis must be
technically sound and based on standard methods of professional practice, it
must also be presented in such a way that it is understandable by decision
makers and elected officials (who often do not have technical backgrounds)
as well as the general public. This is particularly important as we
consider such concepts as level of service, and as we talk about sensitivity
analysis and the relative certainty that we have in our final results.
The major reason for conducting the analyses that makes up this case
study is to provide decision makers with sufficient information about the
performance of the intersection of U.S. 95/Styner Avenue/Lauder Avenue so that
they can decide whether the intersection should be signalized or whether it
should remain stop-sign controlled. The Highway Capacity Manual provides
estimates of delay, volume/capacity ratio, queue length, and other data that
are helpful to decision-makers in making this kind of decision.
should also point out that the Manual on Uniform Traffic Control Devices (MUTCD)
is commonly used to determine if a signal is warranted and in most
jurisdictions is the legal document used to determine many of the
characteristics of a design. However, it is also useful to compare the
forecasted operations of an intersection under different control scenarios. The Highway Capacity Manual provides the tools for such an operational
with Getting Started
Performance Measures The Highway Capacity Manual provides several performance measures for
intersections, both signalized and unsignalized. For both of these
types of intersections, the primary measure of effectiveness is control
delay. Control delay is directly related to level of service, a range
of levels that categorize performance from the point of view of the driver,
or the user of the highway system. Level of service (along with delay)
will be used here to help us determine if the intersection of U.S. 95/Styner
Avenue/Lauder Avenue should be changed from stop sign control to signal
We will also need to consider other performance measures
besides just level of service in order to fully judge the overall
performance characteristics of the intersection. Depending on the type of
control at the intersection, these may include the volume/capacity ratio of
the critical movements at the intersection, the traffic signal cycle length,
and/or queue length considerations.
Analyses We must complete several computations in order to gather the required
information on the performance of the U.S. 95/Styner-Lauder Avenue
under both stop sign and signal control. For convenience, we've
divided the analyses into six parts or problems. We will consider
each problem separately. We will also see that each problem consists
of several separate computations, using the Highway Capacity Manual or other
tools, such as TRANSYT 7F or CORSIM.
we will study the intersection under both stop sign and signal control under
both present and future traffic conditions, for the typical weekday found in
Problem 2, we will consider these same conditions, but
within the context of the U.S. 95 arterial as a whole. Here, we will
particularly focus on the effect that the adjacent intersections have on the
95/Styner-Lauder Avenue intersection, and how a new signal at this
intersection must operate in conjunction with the adjacent signalized
Problem 3, we will consider other time periods,
including traffic volumes during special events at the University of Idaho
and during different times of the year, when the mix of vehicles in the
traffic stream changes.
Problem 4, we will consider how we can forecast the
performance of the U.S. 95/Styner-Lauder Avenue intersection when an
actuated traffic controller is used.
Problem 5, we will consider the operation of the
highway outside the city limits, where it is operating as a two-lane highway.
Problem 6, we will consider the less detailed planning
analysis and what information it can tell us about future operations of the
intersection and the corridor.
with Getting Started
Tools Most of the computations that we conduct in this case study will be with
the Highway Capacity Manual, particularly those chapters that deal with
intersection and arterial operations. This includes Chapter 15 (Urban
Streets), Chapter 16 (Signalized Intersections), and Chapter 17
For some parts of the
analyses, however, and for certain traffic conditions, we will consider the
use of other tools. There are a variety of software tools available
that we could use, and for illustrative purposes we will demonstrate the use
of only two, TRANSYT 7F and CORSIM. TRANSYT 7F is a macroscopic flow
model that consider the progression of traffic platoons from one
intersection to the next. CORSIM is a microscopic traffic model that
considers the movement of individual vehicles along an arterial and the
details of actuated traffic controller operations. Other software packages
could also be applied with equal effectiveness.
Data Three kinds of data are generally needed for traffic analysis.
Demand or volume information specifies the turning movement flows (usually
in vehicles per hour) at each intersection approach. Intersection
geometry includes the number and configuration of lanes at an intersection
and along an arterial. Control conditions include information about
the traffic signal system, including signal timing and phasing data.
The data for each problem can be found at the links provided
below. These links are also provided in each of the problems, as you
need to access them.
TechnicalIssues The computational procedures of the Highway Capacity Manual are complex,
often including a number of default values or assumptions that need to be
understood (and sometimes modified) if the procedures are to be applied
correctly to specific problems.
This case study
presents, and hopefully clarifies, a number of technical issues that often
arise in the application of chapters 15, 16, and 17 of the HCM. A
searchable index of the issues and important key words addressed in this and
other case studies within the HCMAG is provided near the bottom of the table
of contents located in the left margin of this page and labeled as "Search".