Problem 6: Planning
Level Analysis
Printable Version
Thus far, we have
taken an operational perspective in our analysis. Occasionally, when we are
considering future conditions, we are better served by looking at a less
detailed level of analysis, often called a planning level analysis. Let's
take this planning perspective now, considering the twentyyear traffic
projections. All of the intersections on U.S. 95 will be examined to assess
their sufficiency at the end of the twentyyear period. The signalized
intersection quick estimation procedures contained in Appendix A to
Chapter 10 of the HCM 2000 will be applied for this purpose.
The question to
consider now is whether or not the number of lanes at the U.S. 95/StynerLauder Avenue intersection will be sufficient to accommodate the
projected traffic demand over the twentyyear planning horizon. A uniform
increase of twopercent per year will be applied to the current volumes on
all traffic movements. Compounded annually, the twopercent annual
increase will produce a growth factor of 48.6% to apply to the current
volumes.
The quick
estimation method covers all aspects of signalized intersection analysis,
including determination of leftturn treatments, lanevolume computations,
estimation of signal timing plans, calculation of the critical
v/c ratio, and
calculation of average
control delay. The delay calculations use the same
mathematical procedures as the operational analysis method, except that
certain details, such as pedestrian minimum times, are excluded from
consideration. Liberal use is made of assumptions and approximations.
This method is
well suited to analysis of conditions projected over the long term where the
accuracy of the traffic volumes is questionable and immediate implementation
of results is not an issue. The method will be applied to each of the
intersections on U.S. 95 in separate subproblems:
Subproblem 6a: Planning
Analysis at Palouse River Drive
Subproblem 6b: Planning
Analysis at StynerLauder Avenue
Subproblem 6c: Planning Analysis at Sweet Avenue
Subproblem 6d: Planning Analysis at SH8
[
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Discussion [ Continue
] to SubProblem 6a 
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Subproblem 6a: Planning Analysis at Palouse River Drive
The Idaho Transportation Department has
provided historic traffic volume data for U.S. 95 to help determine a historic
growth rate. This data suggests an annual growth rate of 3.4 percent per year
over the past 20 years, and 2.6 percent over the past 10 years.
The City of Moscow has provided planning
model volumes that project 20year traffic growth on the U.S. 95 corridor to be 50
percent over the next twenty years. Based on this information, what is the number of lanes required for the intersections along the
U.S. 95
corridor in twenty years, assuming an annual traffic growth rate along the
corridor of two percent?
In subproblem 6a, we will use the planning
analysis technique to evaluate conditions that occur
when traffic is leaving a football game at the University of Idaho. Demand
is high for about an hour after the conclusion of the game and the U.S. 95
corridor experiences a high level of congestion during this period.
Consider the following:

If leftturn treatments are
unknown, how should they be treated?


How should lanes with shared
through/leftturns be handled?


Will a planning analysis allow complex lane
configurations?


Should right turns be considered a separate movement or
combined with through movements? 
 What is the accuracy of the data used at the planning
level? What does this suggest about the results? 
Discussion:
Take a few minutes to consider
these questions. When you are ready, continue to the next page.
[ Back ] [ Continue ]
with SubProblem 6a 
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Subproblem 6a: Planning Analysis at
Palouse River Drive
The twentyyear projected volumes for this
intersection, adjusted for the
peak hour factor are shown in Exhibit 148.
These projections can be obtained by applying a 2.0 percent growth rate,
compounded annually, to the
existing peakhour
traffic volumes.
Exhibit 148. Palouse River Drive 20Year Projected Volumes 
Approach 
LT (veh/hr) 
TH (veh/hr) 
RT (veh/hr) 
Eastbound 
74 
186 
111 
Westbound 
67 
149 
260 
Northbound 
119 
446 
223 
Southbound 
149 
669 
37 
Each of the steps in the quick estimation method will be
presented in detail in this subproblem. The method involves a series
of five detailed worksheets on which the data are entered and computations
are performed.
The quick estimation steps are as follows
(click on each step to see a more detailed discussion):

Determination of leftturn treatments

Determination of lane volumes

Phasing plan synthesis

Cycle Length determination

Determination of intersection status

Phase time determination

Performance estimation
For purposes of this discussion, the detailed
computations are skipped and the results are summarized in Exhibit 149:
Exhibit 149. Quick Estimation
Summary: U.S.95 at Palouse River Drive 
Direction 
EastWest 
NorthSouth 

Phase 
1 
2 
3 
1 
2 
3 

Movements 
EBWBTH 


SBTHLT 
NBSBTH 

Total 
Critical Volume 
455 


157 
354 

966 
Computed cycle length
in the specified range of 60120 sec: 
60 

Critical v/c ratio
based on the selected cycle length 
0.71 

Status: "Under
Capacity" 
[ Back ] [ Continue ] to SubProblem
6b 
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Subproblem 6b: Planning Analysis at StynerLauder Avenue
A similar evaluation will be performed for the StynerLauder
intersection, but the detailed discussion found in subproblem 6a will not be
repeated here; projected traffic volumes at StynerLauder are much lighter than at Palouse
River Drive.
Similar to the discussion included in subproblem 6a, consider
the following questions for a planning analysis with light volumes
as found at the StynerLauder Avenue intersection:

What sitespecific factors may reduce the accuracy of the
planning level analysis? 

How should the type of signal phasing be decided? 
Discussion:
Take a few minutes to
consider these questions. When you are ready, click
continue below to proceed. [ Back ] [ Continue ] with SubProblem 6b 
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Subproblem 6b: Planning Analysis at StynerLauder
Avenue
The twentyyear projected volumes for the StynerLauder
Avenue intersection,
adjusted for the peak
hour factor are shown in Exhibit 150.
Exhibit 150. 20 Year Projected Volumes at StynerLauder
Avenue 
Approach 
LT (veh/hr) 
TH (veh/hr) 
RT (veh/hr) 
Eastbound 
74 
111 
119 
Westbound 
82 
119 
186 
Northbound 
46 
530 
74 
Southbound 
88 
582 
245 
Click here to see the
crossproduct calculations. None of the leftturn cross products suggests any need for
leftturn protection, so a twophase operation will be used.
Exhibit 151 shows the results from this exercise. Even with
the 20year traffic volume projections, the critical v/c ratio is only 0.52.
These results let us say with great confidence that the existing intersection
configuration will be able to accommodate the anticipated traffic 20 years
from now with no operational problems or congestion.
Exhibit 151. Quick
Estimation Summary: U.S.95 at StynerLauder Avenues 
Direction 
EastWest 
NorthSouth 
Phase 
1 
2 
3 
1 
2 
3 
Movements 
EBWBTH 


NBSBTH 


Critical Volume 
338 


435 


Computed cycle
length in the specified range of 60120 sec 
60 
Total entering vehicles 
773 
Critical v/c
ratio based on the selected cycle length 
0.52 
Status: "Under
Capacity" 
[ Back ] [ Continue ] to SubProblem
6c 
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Subproblem 6c: Planning Analysis at
Sweet Avenue
The U.S. 95/Sweet Avenue intersection has only three
approaches. The planning level analysis of a Tintersection leads to some
questions to consider:

What implications does the lack of a fourth leg have on
the analysis? 

What type of phasing would you expect at a 3legged
intersection? 
 What impact does a protected leftturn have on the
intersection vs. a permitted leftturn movement? 
Discussion:
Take a few minutes to
consider these questions. When you are ready, click
continue below to proceed. [ Back ] [ Continue ] with SubProblem 6c 
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Subproblem
6c: Planning Analysis at Sweet Avenue
The twentyyear projected volumes for this intersection, adjusted for the
peak hour factor, are shown in Exhibit 152.
Exhibit 152. 20Year Projected Volumes at Sweet
Avenue 
Approach 
LT (veh/hr) 
TH (veh/hr) 
RT (veh/hr) 
Eastbound 
354 
0 
279 
Westbound 
0 
0 
0 
Northbound 
152 
643 
0 
Southbound 
0 
886 
97 
This is a Tintersection with no approach for westbound
traffic. The northbound movement has the only left turn opposed by through
traffic. That movement now has a protected left turn phase. Therefore, it is
not necessary to apply the cross product computational step to this subproblem, because the leftturn treatments have already been determined for
all approaches. Instead we must specify the type of protection for each
approach.
The choices are:

Permitted: The left turn moves on a solid green but no protected leftturn phase
is displayed at any point in the cycle.


Protected: The left turn moves only on a green arrow.


Protected
plus permitted: The left turn moves at one point
in the sequence on a green arrow and at another point on a solid green,
yielding to oncoming traffic.


Not
opposed: There is no protected
leftturn phase, but the left turn is never opposed at any point in the
cycle. This choice applies at T intersections, oneway streets, and
intersections with full directional separation (splitphase operation)
between opposing movements.

Discussion:
Which treatment will apply
to each of the approaches at the U.S. 95/Sweet Avenue intersection? When you are ready, click
continue below to proceed. [ Back ] [ Continue ] with SubProblem 6c 
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Subproblem 6c: Planning Analysis at Sweet Avenue
For this example, the Northbound left turn is protected,
the eastbound left turn is not opposed, and the other two left turns do not
exist.
The analysis results are presented in
Exhibit 153. Note that the minimum
cycle length of 60 seconds will accommodate the projected 20year traffic
volumes with no apparent operational problems.
Exhibit 153. Quick Estimation
Summary: U.S. 95 at Sweet Avenue 
Direction 
EastWest 
NorthSouth 
Phase 
1 
2 
3 
1 
2 
3 
Movements 
EBWBTH 


NBTHLT 
NBSBTH 

Critical Volume 
416 


160 
500 

Computed cycle length in
the specified range of 60120 sec 
60 
Total entering vehicles 
1,076 
Critical v/c ratio based on
the selected cycle length 
0.79 
Status: "Under Capacity" 
Notice
also that, even though this is a Tintersection with no westbound approach, the
phasing designation for east west traffic is EBWBTH, indicating the presence
of westbound vehicles. This is an important principle that describes the
way that traffic movements are represented in the Quick Estimation model. The
principle is that,
for the Quick
Estimation method, all movements must exist in the model. Those that don’t
exist on the street are simply assigned zero volume and capacity. This is
an important distinction between the Quick Estimation method and more
detailed traffic models.
As
indicated above, the HCM quick estimation method distinguishes between
protected and protectedpluspermitted leftturn treatments. It is
important to note, however, that the mathematical treatment of the permitted
phase differs from the full operational procedure. The full operational
procedure computes a capacity for vehicles turning left on the permitted
phase, based on advance knowledge of the signal timing plan. The
Quick Estimation procedure does not require this information, and cannot,
therefore, perform the computations at the same level of detail. This is an
essential feature of the tradeoff between the reduced data requirement for
planning level applications and the amount of detail that can be expected in
the results.
[ Back ] [ Continue
] with SubProblem 6c 
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Subproblem 6c: Planning Analysis at
Sweet Avenue
The quick
estimation method takes protected plus permitted phasing into consideration
by subtracting two sneakers per cycle from the leftturn volume. The volume
is not reduced at all for protected only operation, and it is not reduced
below four vehicles per cycle in either case. The four vehicles per cycle
lower limit was imposed to prevent unreasonably short left turn phases from
occurring in the sequence. If the cycle length is not known, the HCM
suggests using the maximum allowable cycle length to provide a conservative
estimate.
For
planning level analysis with projected volumes, many analysts prefer not to
count on the additional permitted phase to provide capacity for left turns
well into the future. There are several reasons, including safety
problems, that a protected left turn supplemented by a permitted phase today
may not be able to retain that option twenty years from now.
Consider
what would happen if the phasing for this example were modified to allow the
left turn to proceed on the solid green for northsouth traffic. The full
operational procedure would assign some additional capacity to the left
turn, thereby reducing the delay, and possibly improving the level of
service.
The quick
estimation method, on the other hand, would see a much smaller difference in
the operation. Based on the maximum cycle length of 120 seconds, four
vehicles per cycle would create a volume of 120 vph. So the leftturn
volume entry would be reduced from 152 vph to 120 vph, but no other benefit
of the permitted phase would be applied. [ Back ] [ Continue ] to SubProblem
6d 
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Subproblem 6d: Planning Analysis at
SH8 Intersection
The same quick estimation will be done for the U.S. 95/SH8
intersection with the following issue to consider:
 If a protected left turn and an exclusive right turn
share a phase, what type of reduction should be applied to the exclusive
right turn volume? 
Discussion:
Take a few minutes to
consider this question. When you are ready to continue, click below to proceed. [ Back ] [ Continue ] with SubProblem 6d 
Page Break
Subproblem 6d: Planning Analysis at SH8 Intersection
The twentyyear projected volumes for this intersection,
adjusted for the peak period factor are shown in Exhibit 154.
Exhibit 154. 20 Year Projected Volumes at SH 8 
Approach 
LT (veh/hr) 
TH (veh/hr) 
RT (veh/hr) 
Eastbound 
73 
774 
687 
Westbound 
254 
0 
574 
Northbound 
0 
657 
305 
Southbound 
0 
0 
0 
This is an unusual intersection that will require careful thought before
applying the quick estimation method. Look again at
Exhibit 14. The departure roadway to the north is
oneway away from the intersection, and the west leg of the intersection is
oneway eastbound. Therefore, there is no westbound through
movement, no northbound left turn, and no southbound traffic at all.

Exhibit
155. Existing Signal Phasing at SH8/U.S. 95 Intersection 
So, the first thing to consider is the type of treatment (permitted,
protected, protected plus permitted or not opposed) to specify for each of
the left turns. Exhibit 155 shows the signal phasing that is currently
being used. The eastbound and westbound movements are fully separated, and
so the "Not Opposed" treatment will apply. While the northbound and southbound
left turns do not exist, they must be specified as permitted because of the
way that the quick estimation method represents traffic movements. For more
detail on this subject, see the discussion presented under
subproblem 6c.
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with SubProblem 6d 
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Subproblem 6d: Planning Analysis at SH8 Intersection
The results of this analysis (critical v/c ratio = 1.23), shown in
Exhibit 156, indicate that the existing geometric configuration and signal
timing would not accommodate the projected volume. This could indicate the
need for intersection improvements, but first we must be sure that the quick
estimation method is representing the operation realistically.
Exhibit 156. Quick Estimation Analysis Summary for SH
8/U.S. 95 
Direction 
EastWest 
NorthSouth 

Phase 
1 
2 
3 
1 
2 
3 

Movements 
EBTHLT 
WBTHLT 

NBSBTH 


Total 
Critical Volume 
847 
675 

359 


1,881 
Computed cycle length in specified range of 60120 sec: 
120 

Critical v/c ratio based on the computed cycle length: 
1.23 

Status: Over capacity 
We should begin by looking at the right turns. The quick
estimation method assumes that the right turns will proceed only on the
concurrent through traffic phases. The phasing plan presented in
Exhibit
155 indicates that both the eastbound and westbound turns are served during
other phases as well. Therefore, these movements may have some capacity that
is not being recognized by the quick estimation method.
The HCM suggests that "when an exclusive rightturn lane
movement runs concurrent with a protected leftturn phase from the cross
street...the total rightturn volume for the analysis can be reduced by the
number of shadowed leftturners." The shadowing movements for the
EB and WB right turns are actually
through movements in this case because of the peculiar intersection
geometrics. The EB right turn is shadowed by the NB through movement, and
the WB right turn is shadowed by the EB through movement. In both cases, the
lane volumes of the shadowing movements exceed the lane volumes of the
shadowed right turns. Therefore, it would be reasonable to remove the EB and
WB right
turns from the analysis.
[
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[
Continue ]
with SubProblem 6d 
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Subproblem 6d: Planning Analysis at SH8 Intersection
The NB right turn is shadowed by the WB left turn, which
takes place from two lanes. From
Exhibit
154, the total WB left turn volume is 254 vph or 127 vph per lane. So,
it would be appropriate to reduce the NB right turn volume of 305 vph by 127
vph to give an effective volume of 178 vph.
The removal of the EB right turn and the reduction of the NB
right turn volume do not affect the results, because the through movement is
the critical movement that governs both of these approaches. On the other
hand, the right turn is the critical movement on the westbound approach. So,
the elimination of that movement will benefit the operation of the whole
intersection.
Exhibit 157 indicates that the removal of the right turns
from the WB approach will reduce the
estimated critical v/c
ratio to from 1.23 to 0.88, indicating a “near capacity” condition. We could
conclude from this analysis that the intersection could probably accommodate
the additional 20 year traffic volumes without the need for geometric
improvements.
Exhibit 157. Quick Estimation
Analysis with Reduced and Eliminated RightTurn Volumes 
Direction 
EastWest 
NorthSouth 

Phase 
1 
2 
3 
1 
2 
3 

Movements 
EBTHLT 
WBTHLT 

NBSBTH 


Total 
Critical Volume 
847 
169 

348 


1,344 
Computed
cycle length in specified range of 60120 sec: 
120 

Critical v/c
ratio based on the computed cycle length: 
0.88 

Status: Near capacity 
A final note on the importance of the specified cycle length
range in the Quick Estimation Method: note that the results for
SubProblem 6a,
SubProblem 6b and
SubProblem 6c
indicated a computed cycle length at the minimum of the specified range and
a “below capacity” status. Our first attempt at Problem 6d indicated a
computed cycle length at the maximum of this range and an “above capacity”
status”. After the right turns were modified, a 120second cycle
length was indicated in Exhibit 157 as the “specified cycle length.”
As a general rule, if the adequacy of the potential capacity of the
intersection is being evaluated, the maximum allowable cycle length should
always be used. If a cycle range is specified, then the results could
be misleadingly pessimistic because of the capacity reduction effect of
shorter cycle lengths.
[
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] [
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] with SubProblem 6d 
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Problem 6Analysis
Simple as they
may be, planninglevel analyses such as the ones conducted in this problem
are powerful tools for the planner, designer, and engineer. Often, the
underlying objectives (i.e., the questions that are being asked and the
reasons the analyses are being undertaken) can be addressed with an adequate
level of accuracy and reliability by planninglevel analysis methods. In
such cases, these firstorder methods usually represent the most cost and
timeefficient means for addressing the underlying objectives of the
investigation, and the increased accuracy available from more sophisticated
and complex methods is neither warranted nor needed.
It is important to
note that critical review and interpretation skills are necessary even when
applying such simple analysis methodologies as the planninglevel techniques
described in this problem. Subproblem 6d, in particular, illustrates how a
careful and critical review of the initial model outputs leads to a
different conclusion than the model initially suggested.
A particular
strength of the planninglevel analysis methodology used in this problem is
its ability to guide the analyst to an appropriate intersection design. The
relationship between the intersection's geometric characteristics and
resultant operational performance measures is clearly delineated by this
method, making it a particularly useful tool for highway designers and
planners as well as operational engineers.
to Problem 6 Discussion 
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Problem 6: Discussion
What did we learn
in this problem? We learned how to apply the Quick Estimation Method (QEM)
to evaluate signalized intersection performance characteristics. We also
learned that the QEM provides a reasonable estimate of intersection
sufficiency on the basis of minimal data, including the approach turning
movement volumes, the approach lane configurations, and the anticipated
method for managing left turns. We saw that the method is a pretty good way
to see whether any geometric improvements will be necessary to accommodate
the anticipated travel demands, particularly when working with future travel
demand estimates that have a significant and inherent amount of uncertainty
associated with them. Finally, we noted how judgment and critical review is
still necessary even when applying the QEM results in order to assure that
appropriate conclusions are reached. [
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] to Case Study 2 
