Problem 2: U.S. 95
Arterial
Printable Version
The problem statement remains nearly the same as in Problem 1: should the intersection of U.S. 95/StynerLauder Avenue be signalized or
left to operate as a TWSC intersection?
But we will now consider the operation of the intersection as
part of the U.S. 95 arterial system. With this context in mind, we will complete
three computations, using the HCM, to get a better picture of the operation of the
intersection under both TWSC and signal control considering the effects of the
adjacent intersections of Sweet Avenue and Palouse River Drive. We will
also consider a fourth subproblem in which we look at some of the issues
involved in signal coordination.
In this problem, you will consider the following issues as you
work through the computations for the three subproblems:

What is the level of service of the
intersection of U.S. 95/StynerLauder Avenue operating as a TWSC intersection
under today's traffic volumes,
considering the effects of the adjacent intersections at Sweet
Avenue and Palouse River Drive? 

What is the level of service of the intersection of
U.S./StynerLauder Avenue operating as a signalized intersection
under today's traffic volumes, considering the effects of the adjacent intersections
at Sweet Avenue and Palouse River Drive? 

What is the level of service of the arterial
segment of U.S. 95 from Palouse River Drive on the south to State Highway 8 on the north? 

Will the location of a new signal at U.S. 95/StynerLauder Avenue affect signal coordination along the
U.S. 95 corridor? 
This problem illustrates some of the
important elements of performing an analysis of a signalized intersection
operating in the vicinity of nearby intersections by addressing the
following issues as they relate to the proposed signal at the U.S. 95/StynerLauder
Avenue intersection:
Subproblem 2a: Effect of upstream
signals on TWSC intersection capacity
Subproblem 2b: Arrival type at
signalized intersections
Subproblem 2c: Arterial (urban street)
analysis
Subproblem 2d: Signal progression and Timespace diagrams
[ Back
] to Problem 1 [ Continue ] to SubProblem 2a 
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Subproblem 2a: Analysis of the
Existing TWSC Intersection Considering the Effects of the Adjacent Intersections
Step 1. Setup
In subproblem 2a, we will evaluate the operational
characteristics of the existing TWSC
intersection at U.S. 95/Styner/Lauder. Here are some issues to consider as
you proceed with the analysis of the existing intersection and its performance
under twoway stopcontrol:

Why and how do upstream signalized intersections affect the
operation of a TWSC intersection? 

What additional data are needed when we consider the
effect of adjacent intersections? 

What is the potential effect of midblock driveways on
these arrival patterns? 
 What computational tools are used to compute the
capacity of a TWSC intersection when considering the effect of adjacent
intersections? 
Discussion:
Take a few minutes to consider these questions. When you are ready, click continue below to proceed.
[ Back ] [ Continue
] with SubProblem 2a 
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Subproblem 2a: Analysis of the
Existing TWSC Intersection Considering the Effects of the Adjacent Intersections
Let's discuss each of these issues and how each affects the operational
analysis that we are about to complete.
Why and how do upstream signalized intersections affect the
operation of a TWSC intersection? Traffic departs a signalized
intersection in wellstructured platoons. These platoons begin to
disperse as they travel downstream from the signalized intersection. When they arrive at a downstream TWSC intersection, the platoon may remain
somewhat intact, depending on the distance from the signalized intersection.
When a TWSC intersection is relatively close to the signalized intersection,
the large gaps that are present between the arrivals of each platoon are
available for use by minor street vehicles. These large gaps generally
have a neutral or positive effect on the TWSC intersection's minor
movements. Consideration
must also be given to the arrival patterns on the major street from the
opposite direction (where there might not be a signalized intersection),
which may have the effect of negating the platoon effects from the
signalized intersection.
You will recall that we are using the procedure from
Chapter 17 of the HCM. However, this model assumes that vehicles arrive randomly or
independently of each other. This random distribution of headways
results in a lower capacity than we would observe from the platooned
arrival condition described above. We should also note that the HCM
procedure does include a means for accounting for the nonrandom effects of
upstream signalized intersections, albeit a fairly rough approximation.
[ Back ] [ Continue
] with SubProblem 2a 
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Subproblem 2a: Analysis of the Existing TWSC Intersection
Considering the Effects of the Adjacent Intersections
What additional data are needed when we consider the
effect of adjacent intersections? The additional data needed are for the
upstream signalized intersection. We need to consider the following
variables:

distance from the signalized intersection to the
subject TWSC intersection 

the progression speed of the through platoon 

the length of the traffic signal cycle 

the volume of the platooned or progressed vehicles 

the
saturation
flow rate of the signalized intersection 

the
arrival
type for major street through vehicles at the signalized intersection 
 the
effective
green time for the signalized intersection 
What is the potential effect of midblock driveways on
these arrival patterns? If there are midblock driveways between the
upstream signalized intersection and the subject TWSC intersection, vehicles
other than those in structured platoons from the signalized intersection
will arrive at the TWSC intersection. These vehicles may reduce the size of
the large gaps that otherwise would be present at the TWSC
intersection. While the HCM model does not account for this effect, you
should keep this in mind if the intersection you are studying has midblock
driveways.
What
computational tools are used to compute the capacity of a TWSC intersection
when considering the effect of adjacent intersections? We will use the
same procedure from Chapter 17 of the HCM for TWSC intersections. This time,
however, we will consider the upstream signal module that is included in
this procedure.
[ Back ] [
Continue ] with SubProblem 2a 
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Subproblem 2a: Analysis of the
Existing TWSC Intersection Considering the Effects of the Adjacent Intersections
Step 2. Results
The HCM procedure produces the following results for each minor stream movement:

capacity of the movement and
the capacity of the lane or lanes 

delay for the movement and the
weighted average delay for each lane 

95thpercentile queue for each lane 
 level of service for each lane 
These results are summarized in Exhibit 120, for the
existing volumes.
Exhibit 120.
U.S.
95/StynerLauder Avenue (Dataset9)
Delay,
Queue Length, and Level of Service  Existing Volumes 
Approach 
NB 
SB 
Westbound 
Eastbound 
Movement 
1 
4 
7 
8 
9 
10 
11 
12 
Lane configuration 
L 
L 
L 

TR 
L 

TR 
v (vph) 
31 
59 
55 
205 
50 
155 
C (m) (vph) 
1024 
1163 
170 
359 
134 
329 
v/c 
0.03 
0.05 
0.32 
0.57 
0.37 
0.47 
95% queue length 
0.09 
0.16 
1.31 
3.39 
1.56 
2.41 
Control delay 
8.6 
8.3 
36.0 
27.6 
47.0 
25.3 
LOS 
A 
A 
E 
D 
E 
D 
Approach delay 
 
 
29.3 
30.6 
Approach LOS 
 
 
D 
D 
Discussion:
If we review the results
produced when we consider the effect of arrival patterns from the Sweet
Avenue signal (above), we see that there is no difference from our
previous analysis. Why?
[ Back ] [ Continue
] with SubProblem 2a 
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Subproblem 2a: Analysis of the Existing TWSC Intersection
Considering the Effects of the Adjacent Intersections
Exhibit 121. Analysis of the U.S.95/StynerLauder
intersection with effects of adjacent intersections (Dataset9) 
Subproblem 
NB LT 
SB LT 
WB LT 
WB TH/RT 
EB LT 
EB TH/RT 
Previous analysis
(subproblem 1a) 
8.6 
8.3 
36.0 
27.6 
47.0 
25.3 
Current analysis
(subproblem 2a) 
8.6 
8.3 
36.0 
27.6 
47.0 
25.3 
The answer lies in
the distance between the TWSC intersection at Styner and the signalized
intersection at Sweet Avenue. When a platoon leaves Sweet Avenue and travels
1,070 feet to Styner, it begins to disperse. Platoon
dispersion means that the headways between vehicles begin to approximate
a random distribution, which is what the TWSC intersection capacity model is
based on. Thus, for this case, we see that the arrival patterns from the
Sweet Avenue traffic have no effect on the capacity of the TWSC intersection
at Styner/Lauder.
Discussion:
When would a signal make a
difference in the arrival patterns at the downstream TWSC
intersection? Continue to the next page for more discussion on this topic.
[ Back ] [
Continue ] with SubProblem 2a 
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Subproblem 2a: Analysis of the
Existing TWSC Intersection Considering the Effects of the Adjacent Intersections
Exhibit 122 shows a plot of capacity for the EB
through and right turn movement at the U.S. 95/StynerLauder intersection as
a function of the distance away from an upstream signalized
intersection. The last point on the right of Exhibit 122 shows a capacity of 329 veh/hr for
a distance of 1,070 feet, which is the distance between Sweet Avenue and StynerLauder
(Dataset).
In fact, we can see that for any distance greater than about 600 feet, there
is no capacity increase.
But when we get closer than 600 feet to the signalized
intersection, the platooning begins to have an effect on the capacity of
this movement. At 250 feet from the intersection, the capacity
increases by 20 percent to 396 veh/hr. The delay is also reduced by 20
percent, from 25 sec/veh to 20 sec/veh.
So, while there may be some effect of upstream signals in
some situations, this effect does not show up in this particular problem.
It is important to point out that the method we've just used from the HCM to account for the effect of upstream signals is an approximation. Where the effect of the upstream signals is more important to the final
result than in this example, other techniques are available for
estimating this effect. These techniques are more microscopic, require
more input data and effort to apply, but may not give any more accurate
answer, unless more refined data is provided. They can, however, provide further
insight and are warranted for use in some situations.
We will next consider, in subproblem 2b, whether arrival
patterns of Sweet Avenue vehicles have an effect on the StynerLauder
intersection if the latter intersection were to be signalized.
[ Back ] [ Continue
] to SubProblem 2b 
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The graphic below shows a plot of
capacity vs. distance downstream from a signalized intersection for the
conditions given in subproblem 2a. We assumed for this problem
that, for the upstream signalized intersection, the cycle length was
xx
seconds and the green time was xx seconds. 

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Subproblem 2b:
Analysis of the Signalized Intersection Considering the Effects of Adjacent Intersections
Step 1. Setup
We will now look at the operation of U.S. 95/StynerLauder Avenue as a signalized intersection and consider the effects
of the adjacent signalized intersection at Sweet Avenue. Sweet Avenue
is located 1,070 feet to the north of StynerLauder Avenue and is the
main southern entrance to the University of Idaho for university students,
staff, and faculty. Vehicles traveling south on U.S. 95 from Sweet
Avenue often arrive in platoons during the peak period. While we
didn't see an effect of platooning on the operation of StynerLauder Avenue as a twoway, stopcontrolled intersection, there may be an effect if
the operation is controlled by a traffic signal. What is the nature of
this effect?
Discussion:
Take a
few minutes to consider how the traffic signal at Sweet Avenue might affect
the operation of StynerLauder Avenue, if the latter intersection
were to be signalized.
[ Back ] [ Continue
] with SubProblem 2b 
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Subproblem 2b:
Analysis of the Signalized Intersection Considering the Effects of Adjacent Intersections
The major effect that the traffic signal at Sweet Avenue
has on the adjacent intersection at StynerLauder Avenue, if the
latter intersection were to be signalized, is related to the pattern of vehicle
arrivals. If the two signals are interconnected, with a fixed offset,
the arrival pattern would be nearly the same during each cycle. The
key issue is whether the platoons from Sweet Avenue arrive primarily during
the green phase or primarily during the red phase, of if they arrive
randomly during the cycle.
Preliminary studies indicate the offset of the northsouth
green phase on U.S. 95 at StynerLauder Avenue is 20 seconds. This would result in favorable progression for the southbound traffic, or
arrival type 4. All other factors that we considered in subproblem 1b would remain the same for this analysis.
[ Back ] [ Continue
] with SubProblem 2b 
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Subproblem 2b:
Analysis of the Signalized Intersection Considering the Effects of Adjacent Intersections
Step 2. Results
The effect of the arrival pattern of vehicles from the Sweet
Avenue intersection to the StynerLauder Avenue intersection depends on
when platoons from Sweet Avenue arrive at Styner/Lauder. The HCM
classifies the way in which platoons arrive at a signalized intersection
according to arrival type. Exhibit 123 shows the results of the delay
calculations for this problem, assuming three arrival types (2, 3, and 4). Again, for this problem, we've found
from field studies that arrival type 4 best describes the
conditions likely to be present once the intersection is signalized.
It should be evident even before we do any analysis that
varying the arrival type from Sweet Avenue will have no effect on the eastbound
approach, the westbound approach, or the northbound approach, since none of
these approaches are affected by the Sweet Avenue signal.
Exhibit
123. U.S.95/StynerLauder
Signalized Intersection Analysis with Varied Arrival Types (Datasets) 
Arrival Type 
EB 
WB 
NB 
SB 
LT 
TH/RT 
LT 
TH/RT 
LT 
TH/RT 
LT 
TH/RT 
2 
19.1 
20.7 
18.9 
22.9 
5.7 
6.1 
7.9 
8.8 
3 
19.1 
20.7 
18.9 
22.9 
5.7 
6.1 
5.9 
6.6 
4 
19.1 
20.7 
18.9 
22.9 
5.7 
6.1 
3.8 
4.2 
However, we do see some effect for the southbound
approach. For the case in which we don't consider the Sweet Avenue signal (see
Exhibit
111), the average control delay is computed to be 6.6 seconds per vehicle for the
through and right turning traffic. This is shown as arrival type 3 in the
table. For our projected condition, with favorable progression and arrival
type 4, the delay is reduced to about 4 seconds per vehicle for this movement. While this is not a significant decrease, it does represent some effect for
traffic on this approach. For purposes of comparison, we have also shown a
less favorable progression condition, arrival type 2. Here, the average
control delay increases to nearly 9 seconds per vehicle for the through and right
turning traffic.
[ Back ] [
Continue
] to SubProblem 2c 
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Subproblem 2c:
Analysis of the U.S. 95 Arterial
Step 1. Setup
We now turn to a view of the U.S. 95 arterial, of which the StynerLauder Avenue intersection is one part. You can review the physical
layout of the intersection in Exhibit
16. On the
northern end is the intersection of U.S. 95 and State Highway 8. The
Sweet Avenue intersection is located 560 feet to the south of the State
Highway 8 intersection. It handles about 1,600 vehicles during a
typical afternoon peak hour.
StynerLauder Avenue is located 1,070 feet south of
Sweet Avenue, while the Palouse River Drive intersection is located 2,410
feet further south. The average speed on U.S. 95 is 35 miles per hour.
Discussion:
How do we
determine the operational performance of an urban street or arterial? Which tools should be used for the analysis of an urban street? What
data are required for the analysis? Take a few minutes to consider
these questions. When you are ready, proceed to the next page.
with
SubProblem 2c 
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Subproblem 2c:
Analysis of the U.S. 95 Arterial
Several tools are available to analyze the operations of
urban streets or arterials. Microscopic simulation programs that go
beyond the capabilities of HCM procedures can be used if we have oversaturated conditions, closely
spaced intersections where queues from one or both intersections
interact, or if we want to study specific actuated controller parameters. Macroscopic
simulation/optimization programs are also available for use, either in
conjunction with or in lieu of the HCM procedures, to
assess the arterial operations for undersaturated operations. Here, we
will use the HCM urban street procedure from Chapter 15 of the HCM.
What data are needed for application of the HCM
methodology? In conducting an urban street analysis, you need first to identify the major
cross streets (signalized intersections) along the arterial, the distances
between these cross streets, and the free flow speed along the arterial. The free flow speed is the average travel speed that vehicles would operate
without being affected by signalized intersections or other vehicles. You also must determine the urban street classification, using Exhibits 104
and 152 from the HCM. Finally, the traffic signal data for each
signalized intersection is needed. In effect, what we must do is
conduct an operational analysis for each signalized intersection and
use the estimates of control delay that result from each intersection
analysis.
[ Back ] [ Continue
] with SubProblem 2c 
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Subproblem 2c: Analysis of the
U.S. 95 Arterial
Step 2. Results
The HCM urban streets methodology produces travel speed
and level of service for each arterial segment, as well as for the entire
urban street. But to get an idea of the effect of a signal at the StynerLauder Avenue intersection, we must also estimate the speed and level
of service for the urban street, without the new signal at StynerLauder.
Exhibit 124 summarizes these results.
Exhibit
124. U.S.95
Urban Street Analysis (Dataset12,
Dataset13) 
Segment 
With signal at
StynerLauder
Travel speed (LOS) 
Without signal at
StynerLauder
Travel speed (LOS) 
Sweet to StynerLauder 
22.4 mi/hr (C) 
29.3 mi/hr (B) 
StynerLauder to Palouse River Drive 
27.1 mi/hr (C) 
Overallurban street 
25.4 mi/hr (C) 
29.3 mi/hr (B) 
We can see that the addition of the signal at StynerLauder
reduces the travel speed by about 4 mph, or about 13 percent. The
level of service is reduced from B, without the signal, to C, with the
signal.
[ Back ] [ Continue
] to SubProblem 2d 
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Subproblem 2d:
Effects of a Signal on an Existing Coordinated System
In subproblem 2c, we
analyzed the operation of a portion of the arterial, of which the StynerLauder
intersection is a part, using the HCM methodology. But we must also
consider another factor in the decision to signalize this intersection. How
will the new signal at U.S. 95/StynerLauder Avenue affect signal
coordination along the U.S. 95 corridor? To answer this question, we must
examine the three coordinated intersections as a system (see
Exhibit 125).
The first task is to
determine whether the existing system is coordinated (i.e. has a common cycle length).
If the signals currently operate in an uncoordinated mode, we will have to establish coordination between them by choosing a common cycle length. There are three important considerations that
should be taken into account when selecting an appropriate cycle length:

Individual intersection timing requirements. Intersection
phasing, pedestrian timing, and other factors dictate the lower bound for
any common
cycle length. As an example, consider the U.S. 95/SH 8 intersection, which
currently operates under split phasing to accommodate the existing lane
configuration. Split phasing strategies typically require a higher cycle
length than if the left turns were protected or permitted.
The effect is to cause this intersection to have the highest cycle length
requirement of the intersections on the arterial. Therefore, the SH8 cycle
length establishes the lower bound of the common cycle length requirement.

Distance between intersections. Closelyspaced intersections such as
Sweet and SH 8 often benefit from lower cycle lengths, which allow for
better queue management characteristics. The distance between intersections and queue storage
considerations are key in the development of
signal timing plans. While lower cycle lengths may sacrifice progression
efficiency, they can still perform better, because queue spillback and system
delay (especially on the side street) will be minimized.

Potential for cycle failures.
When the cycle length is too short, cycle failures will occur, and the
responsive operation that is characteristic of low cycle lengths may be
offset by increased delay on movements where demand exceeds capacity. In
such cases, somewhat longer cycles may actually achieve better progression—for example, where arterial
green phases must be displayed more or less simultaneously.
[ Back
] [ Continue ] with
SubProblem 2d 
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Subproblem 2d:
Effects of a Signal on an Existing Coordinated System
Finding the optimal common cycle length can be achieved by
using any of several traffic models that take all of the
previouslyidentified factors into
consideration. The HCM, however, does not include a methodology that can be
used to make this determination. For the purposes of this discussion we will choose a 90second
cycle; this choice will accommodate a progression band through the
intersections that is no smaller than the shortest arterial green phase,
which, in this case is found at SH 8.
Now, if we extend the time–space diagram (see Exhibit 126) to include the new intersection at StynerLauder, we can determine the effect of a signal at this intersection
on the entire coordinated system. Note that the distance to StynerLauder
dictates an alternating relationship between the green phases with the
adjacent intersection. Similarly, the close spacing of the SH 8 and Sweet
intersections dictates a moreorless simultaneous green phase strategy for
the through movements. The simple twophase operation at StynerLauder
provides a longer green phase on the arterial (NB and SB approaches), so the additional signal is
able to fit into the progression scheme without encroaching into the bands
of progression that currently exist between the two signals.
The conclusion
that we can draw from this is that it is possible to signalize the StynerLauder intersection without a significant adverse effect on the
driverperceived progression, even though the arterial street analysis
conducted in subproblem 2c
suggests that travel speed
will be impacted by 13 percent.
Exhibit 126. Signal Coordination Time/Space Diagram
[ Back ] [
Continue ] to
Problem 2 Analysis 
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Problem 2: Analysis
It is now
useful to bring together the results of the three subproblems that we
considered as part of problem 2. In this problem, we considered the
question of whether or not to signalize U.S. 95/StynerLauder Avenue not as an isolated one,
in which we look at only the conditions at the intersection, but rather
in the context of the U.S. 95 arterial and the intersections adjacent to StynerLauder. We learned in subproblem 2a
that the flow patterns from the adjacent intersections (specifically Sweet
Avenue) do not affect the capacity of the StynerLauder Avenue intersection when it
is operating with stopsign control. The distance between Sweet Avenue
and StynerLauder is great enough that the platoons from Sweet Avenue have
dispersed sufficiently so that the capacities of the StynerLauder approaches
are not affected. We learned in subproblem 2b that
if the intersection of StynerLauder were signalized, there would be some
effect on the delay of the U.S. 95 traffic at the intersection. This
reduction in delay over the conditions that we considered in problem 1
results from the degree of coordination that can be achieved between the
Sweet Avenue and StynerLauder Avenue intersections. Both
results continue to leave open the opportunity to signalize the StynerLauder
intersection. We found in subproblem 2c, however,
that when we apply the HCM urban street methodology, the benefits to the side streets (Styner
and Lauder approaches) must be traded off against a reduction in level of
service along the U.S. 95 arterial. The level of service before StynerLauder
is signalized is estimated to be B. After the
signal is installed, even with the seemingly acceptable delay to U.S. 95
traffic, the level of service for this arterial segment would be reduced to C,
because the travel speed on
the arterial is reduced from 29 mi/hr to 25 mi/hr.
There are other ways to minimize the impacts of a new signal on arterial
operations and the throughtraffic travel times. One of these ways is to
coordinate the system of signalized intersections within the arterial
section, and the effect of this was demonstrated in subproblem 2d.
It is also possible to minimize these impacts by restricting the
nonarterial green time at the new StynerLauder/U.S. 95 traffic signal. A
methodology for accomplishing this latter approach is further discussed in Problem 4. [ Back ] [ Continue ] to Problem 2 Discussion 
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Problem 2: Discussion
What is
next to consider in this problem, after looking at more of a system context
for the operation of the intersection and the arterial? Note that we
have focused our analysis so far on just the afternoon peak period. It
is common in traffic analysis to look at the weekday peak periods to
ensure that the traffic system is operating in an acceptable manner during
this time. However, it is also important to ask ourselves if there are
other time periods that require consideration? The
answer here is yes. This is a university town; and in addition to
normal weekday peak periods of travel, there are a number of other traffic
patterns that we should consider. The university has a number of
special events during the year, each attracting a large number of visitors
and attendees at the events. In addition, the vehicle mix in the
traffic stream varies during the year, a fact that should also be assessed. In problem 3, we will consider these travel patterns and
determine if they have an effect on our decision to signalize the
intersection of U.S. 95/StynerLauder Avenue. [
Back ]
[ Continue ]
to Problem 3 
