Problem 2: Reitz Union Drive Intersection Improvement
Strategies
Printable Version
Continuing our
analysis of the Reitz Union Drive intersection, we will now look at some
additional options under signal control. We can first analyze operations of
the improved intersection with no northbound approach and two exclusive
rightturn lanes southbound, using an actuated, twophase signal with a
120second cycle. Then, we can investigate improvements to this design to
quantify their effects as input to choosing alternatives to potentially
minimize congestion.
Continuing to use future
traffic projections and the improved geometric conditions suggested in
SubProblem 1c, we can compute the
delays and level of service at this signalized intersection, assuming that
it is operating under
fullyactuated control. As we work through these computations, we will be
able to investigate several aspects of an actuated signal.
We can first look at
some options to test phasing alternatives to accommodate pedestrians. Such analyses will allow us to quantify the effects of these signal modifications
toward resolving these issues using:

minimum
green times to accommodate pedestrians 

effects
of introducing an exclusive pedestrian phase 
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Subproblem 1c [ Continue ] with Problem 2 
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Problem 2: Reitz
Union Drive Intersection Improvement Strategies
Next, we will consider
an option to improve the phasing at this intersection by investigating its
operation with existing phasing to identify:

deficiencies
in capacity and delay by movement 

phasing
modifications to alleviate these movement deficiencies 
Finally, we can
investigate semiactuated control for potential coordination with adjacent
signals and what effect it has on the overall operation in
subproblem 2c
by looking at:

unit
extension and kvalue versus
arrival type and progression factor 

double
cycle option to compare overall delay and level of service 
We will compare the
overall operation of the signalized intersection operating with and without
improved phasing and timing, as well as actuated versus semiactuated
control to better recommend alternative solutions.
In Problem 2, we are
looking at strategies to improve the intersection, mostly as an isolated
signal with the addition of the traffic generated by the new parking
structure, but we are introducing the idea of semiactuated control for
potential coordination with the adjacent intersections.
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[ Continue ] with Problem 2 
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Problem 2: Reitz Union Drive Intersection
Improvement Strategies
We can set up this
analysis by using the data from Problem 1 to analyze the actuated signal
control of the improved intersection with projected traffic in a standard
HCM application using the procedures in Chapter 16 for signalized
intersections. The operational analysis of a signalized intersection under
actuated control requires the following typical data:

peakhour
turning movement volumes and
peakhour factors 

pedestrian
and bicycle flow rates 

pedestrian
walking speed, travel distance, and crosswalk width 

lane
numbers, widths, and usage 

queue
spacing and storage signal phasing, timing, and clearance data 

number of
approach grades, heavy vehicles, parking, and bus stop influence 
The critical data for
this analysis are shown in the following table:
Exhibit 522. Input Data for Analysis of Actuated
Control: Museum Road/Reitz Union 

EB 
WB 
SB 
L 
T 
R 
L 
T 
R 
L 
T 
R 
Lanes 
1 
1 
 
 
1 
S 
1 
 
1 
Widths 
12 
12 
 
 
12 
 
12 
 
12 
Storage 
250 
500 
 
 
500 
 
150 

150 
Volumes 
379 
670 
 
 
597 
178 
204 
0 
484 
Peds 



Flow 
Speed 
Dist 



WB Only 
 
 
 
500 
4 
40 
 
 
 
Intersection Phasing 

Phase 1 

Phase 2 
G = 70 sec 
G = 40 sec 
Y = 4 sec 
Y = 4 sec 
R = 1 sec 
R = 1 sec 
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Problem 2 
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Problem 2: Reitz
Union Drive Intersection Improvement Strategies
Using this data to
work through the HCM procedures will yield for each movement and approach:

capacity 

v/c ratio 

queue
storage ratio 

delay 

level
of service 
Results of our analysis, including overall intersection delay and level of
service, are shown in the following table:
Exhibit 523. Museum Road at Reitz
Union Drive (120sec cycle) 

EB 
WB 
SB 
L 
T 
R 
L 
T 
R 
L 
T

R 
Volumes 
379 
670 
 
 
597 
178 
204 
 
484 
Queues 
46 
30 
 
 
38 
38 
11 
 
16 
Delay 
219 
18 
 
 
22 
22 
31 
 
33 
LOS 
F 
B 
 
 
C 
C 
C 
 
C 
Intersection 
Delay 
53.4 
LOS 
D 
Remember, since the HCM methodology assumes all
lanes have adequate storage when computing delay, you must check the
backofqueue and queue storage ratio for each lane group to determine if
there are queue storage problems. (A queue storage ratio greater than 1.0
indicates estimated queues are greater than available storage).
With this analysis of the existing conditions at the Reitz
Union Drive intersection, we will address intersection improvement
strategies in the ensuing subproblems.
Subproblem
2a:Analyzing the effects of pedestrians
Subproblem
2b:Analyzing the effects of signal phasing
Subproblem 2c:Analyzing the effects of coordination
Discussion: Summary of Problem 2 results
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Subproblem 2a 
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Subproblem 2a:
Analyzing the Effects
of Pedestrians
Step 1. Setup
Because of the large
number of pedestrians at this intersection, we might want to consider an
exclusive pedestrian phase. We have resolved part of the pedestrian
conflicts with vehicles by closing the northbound approach to vehicular
traffic in
Problem 1. However, the northeast movement of pedestrians from
parking to classes suggests a diagonal crossing might be worth some
consideration.
Assuming we need to
maintain the 120second cycle, we need to compute the pedestrian green time
for the diagonal movement crossing. Using a crossing distance of 50 feet, a
crosswalk width of 10 feet and a 4feetpersecond walk speed, the minimum
pedestrian time is calculated to be about 20 seconds. This assumes 500
pedestrians in thirty 120second cycles per hour, or 17 per interval in the
following formula from HCM Equation 162:
Incorporating this
value into an exclusive pedestrian phase yields the following timing for the
intersection:

Phase1 

Phase2 

Phase3 
G = 73 sec 
G = 17 sec 
G = 20 sec 
Y = 4 sec 
Y = 4 sec 
Y = 0 sec 
R = 1 sec 
R = 1 sec 
R = 0 sec 
Discussion:
Take
a few minutes to review the phasing plan with the exclusive pedestrian
phase. By moving the pedestrians to their own phase, will they be assigned
more or less time than they were before? What benefits are derived by
assigning pedestrians with an exclusive phase? Click continue when you are ready
to proceed.
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Subproblem 2a 
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Subproblem 2a:
Analyzing the Effects
of Pedestrians
Step 2: Results
To evaluate the
effects of this signal timing strategy, we will use the Chapter 16 HCM
operational methodology. Running the analysis
for a 120second cycle results in the queues, delays, and LOS
values shown in Exhibit 524:
Exhibit 524. Museum Road at
Reitz Union Drive (Exclusive Ped Phase) 

EB 
WB

SB 
L 
T 
R 
L 
T 
R 
L 
T 
R 
Volumes 
379 
670 
 
 
597 
178 
204 
 
484 
Queues 
42 
28 
 
 
36 
36 
16 
 
32 
Delay 
168 
16 
 
 
19 
19 
72 
 
188 
LOS 
F 
B 
 
 
C 
C 
E 
 
F 
Intersection 
Delay 
77.5 
LOS 
E 
While pedestrians are
accommodated more efficiently and more safely, the intersection performance
is worsened under this scenario, at least with respect to the LOS criterion.
On the southbound approach, LOS drops from C to E/F. Since pedestrians are restricted to crossing only the northbound
and westbound approaches (with the northbound approach closed to vehicles),
the need for an exclusive pedestrian phase is probably minimal, especially
with the deterioration of the intersection efficiency using this design.
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Subproblem 2b 
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Subproblem 2b:
Analyzing the Effects
of Signal Phasing
Step 1. Setup
Remember from
our initial analysis of the Reitz Union Drive improvement strategy (see
Exhibit 523) that the most urgent deficiency is the
eastbound leftturn movement, with an estimated delay in excess of 200
seconds per vehicle. The first mitigating option that would be the easiest
to implement and the least costly would be to simply modify the signal
phasing. We can test if providing a signal phase to alleviate the
saturated movements might reduce the delay on those movements,
being cognizant of the effects on the overall intersection.
Discussion:
Take
a few minutes to determine a phasing plan that would reduce the delay of the
critical movements. Click continue when you are ready to proceed.
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Subproblem 2b 
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Subproblem 2b:
Analyzing the Effects
of Signal Phasing
Step 2: Results
Referring once again
to the results of our initial analysis (see
Exhibit 523),
we see that there may be an
opportunity to combine improvements through phasing design, since the two
most deficient movements are the eastbound left and the southbound right.
This allows us to introduce a phase that implements a leading protected
leftturn phase eastbound that can run concurrently with a protected
rightturn phase southbound as shown here:

Phase1 

Phase2 

Phase3 
G = 10 sec 
G = 48 sec 
G = 12 sec 
Y = 4 sec 
Y = 4 sec 
Y = 4 sec 
R = 1 sec 
R = 1 sec 
R = 1 sec 
This phasing provides
more protected time to the most deficient movements to improve the overall
efficiency of the intersection, as illustrated in the results below:
Exhibit 525.
Museum Road at Reitz Union Drive (Leading EB Left) 

EB 
WB 
SB 
L 
T 
R 
L 
T 
R 
L 
T 
R 
Volumes 
380 
670 
 
 
595 
180 
205 
 
485 
Queues (veh) 
11 
19 
 
 
34 
34 
15 
 
17 
Delay 
21 
6 
 
 
17 
17 
55 
 
38 
LOS 
C 
A 
 
 
B 
B 
E 
 
D 
Intersection 
Delay 
22.0 
LOS 
C 
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Subproblem 2b:
Analyzing the Effects
of Signal Phasing
Let's compare the results of this analysis (Exhibit 525)
with our initial assessment (Exhibit 523).
The eastbound left delay went from 219 sec/veh to 21 sec/veh while
maintaining reasonable performance levels for other movements.
The westbound movements
were slightly improved (LOS C to B) with some deterioration for southbound
movements (LOS C to D/E). The overall intersection improved from a delay of
53 sec/veh (LOS D) to a delay of under 22 sec/veh (LOS C). These results
suggest that the added phase offers an overall improvement to the operation
of the intersection at a level that warrants its implementation.
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Subproblem 2c 
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Subproblem 2c:
Analyzing the Effects
of Coordination
Step 1. Setup
The previous
analyses assumed fully actuated signal control. With adjacent signalized
intersections located close by, coordination would be
desirable. This Subproblem provides an opportunity to compare the effects of
operating the Museum Road/Reitz Union Drive signal as fully actuated with
the effects operating it under semiactuated control. A constant cycle
length can be maintained if the intersection is operated under
semiactuated control, thereby allowing coordination with the other signals along Museum Road. In an HCM
analysis, the factors affected in this comparison are
arrival type,
progression factor,
unit extension, and the kvalue.
Discussion:
Take
a few minutes to identify why the factors mentioned above will be affected
in this comparison. Click continue when you are ready to proceed.
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Subproblem 2c:
Analyzing the Effects
of Coordination
Step 2: Results
The
arrival type is 3
for all movements under fully actuated control to model random arrivals.
This results in a progression factor (HCM Equation 1610) of 1.00 for all
approaches, which means the first term of the delay equation (HCM Equation
169) for uniform delay (d_{1}) is not adjusted for coordination.
Under fully actuated
control, the HCM procedures account for how responsive an actuated movement
reacts to traffic by using the unit extension. This value represents how
long (in seconds) a detector must be vacant before the controller will end
the phase ("gap out"). In the HCM, the unit extension is used to determine
the kvalue for use in the delay equation (for incremental delay, d_{2}).
So, while fully actuated control does not lower d_{1}, it does lower
d_{2}.
Conversely, under
semiactuated control, the reverse is true. Since the major street through
movements must be pretimed to accommodate coordination, the arrival type can
vary, based on the degree of coordination provided. Under most situations,
arrival type 4 is used for normal coordinated systems. (Arrival type 5 could
be used in especially well coordinated systems like for oneway streets).
Using arrival type 4 for both the eastbound through movement and westbound through
and rightturn movements in this case results in progression factors from HCM Exhibit 1612, based on the green (g/c) ratios but always values less
than 1.00 to account for improvements in delay to these movements created by
the coordination provided. The progression factor modifies the effects of
uniform delay, d_{1}.
However, under
semiactuated control, the unit extension value for the eastbound through
movement and the westbound through and rightturn movements will be ignored
since these movements are under pretimed operation, resulting in a kvalue of 0.50. This will not
lower the d_{2} value, so we have a tradeoff between these two
control strategies.
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Subproblem 2c: Analyzing the Effects
of Coordination
Our task here will be
to compare these results to see which combination provides the better
overall efficiency. We can make two runs: one with all actuated movements,
arrival types of 3 and unit extension values of 3.0 seconds, and the other
with eastbound through movements and westbound through and rightturn movements
coded as pretimed, using arrival type of 4, with the unit extension values
to be ignored. The results of these two runs are presented in Exhibit 526
so that we can compare the affected approaches and the overall intersection
operations:
Exhibit 526. Comparison of Fully Actuated to
Semi Actuated Control 

Fully Actuated 
SemiActuated 
EBT 
WBT 
WBR 
EBT 
WBT 
WBR 
Volumes 
670 
597 
178 
670 
597 
178 
Arrival Type 
3 
3 
3 
4 
4 
4 
Progression Factor 
1.00 
1.00 
1.00 
0.24 
0.44 
0.44 
d_{1} 
6.0 
15.1 
15.1 
5.6 
13.2 
13.2 
PF * d_{1} 
6.0 
15.1 
15.1 
1.3 
5.8 
5.8 
Unit Extension 
3.0 
3.0 
3.0 
 
 
 
kValue 
0.11 
0.27 
0.27 
0.50 
0.50 
0.50 
d_{2} 
0.3 
2.0 
2.0 
1.2 
3.1 
3.1 
Movement Delay 
6.2 
17.2 
17.2 
2.5 
8.9 
8.9 
Movement LOS 
A 
B 
B 
A 
A 
A 
Intersection Delay 
22.0 
16.6 
Intersection LOS 
C 
B 
As you can see, the
unit extension creates kvalues of less than 0.50, which lowers d_{2}.
But, arrival type values of 3 result in progression factors of 1.00, with no
effect on d_{1}. However, under semiactuated control, the kvalue
stays at 0.50, not affecting d_{2}, but the progression factor is
lower which reduces the d_{1} term. Overall, we can see that the
combined effects of semiactuated control on the d_{2} term outweigh
those of fullyactuated control on the d_{1} term.
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Subproblem 2c: Analyzing the Effects of Coordination
One last option we
might consider, since most of these runs have assumed a 120second cycle, is to
investigate the possibility of a shorter cycle. However, in order to
maintain the ability to coordinate with the other signals along the arterial
(which we are assuming to be running the 120second cycle), we must confine
ourselves to a 60second cycle. This is because the cycle length we use at
Museum/Reitz Union must maintain an integer relationship with the 120 second
cycle that controls other intersections along Museum Road. As examples,
therefore, it could be 30, 40, or 60 seconds in duration. However, we must
be cognizant of pedestrian crossing time requirements, and this is likely to
eliminate anything less than 60 seconds. Operating the Museum Road/Reitz
Union Drive intersection on a 60second cycle while the remainder of the
system operates on a 120second cycle is commonly referred to as a
doublecycle option; it has the potential to lower delay at our intersection
of interest (because of the lower cycle length) while still maintaining the
benefits of coordination with upstream signals. The results of this
comparison are shown in Exhibit 527.
Exhibit 527. Museum Road at
Reitz Union Drive (120 vs. 60sec cycle) 

EB 
WB 
SB 
L 
T 
R 
L 
T 
R 
L 
T 
R 
Volumes 
379 
670 
 
 
597 
178 
204 
 
484 
Queues (120) 
9 
8 
 
 
22 
22 
14 
 
17 
Queues (60) 
7 
5 
 
 
20 
20 
9 
 
9 
Delay (120) 
9 
3 
 
 
9 
9 
52 
 
39 
Delay (60) 
15 
2 
 
 
14 
14 
40 
 
19 
LOS (120) 
A 
A 
 
 
A 
A 
D 
 
D 
LOS (60) 
B 
A 
 
 
B 
B 
D 
 
B 
Intersection (120) 
Delay 
16.6 
LOS 
B 
Intersection (60) 
Delay 
14.0 
LOS 
B 
Comparing the results between the 120second and 60second cycles
illustrates a couple of points. Lower cycle lengths generally result in
shorter delays overall, even though capacities go down. Also, the queues are
reduced substantially because the red time per phase is less, reducing the
time available for queues to lengthen. This is a good strategy where storage
lengths are limited, as is the situation in this particular case study. The overall intersection delay was
reduced and queues lowered for every movement by between 10% and 50%. Another
byproduct of this strategy is that the large number of pedestrians will not
need to wait as long before receiving a WALK indication.
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Discussion
In this problem we
evaluated an implementation of an exclusive pedestrian phase, alteration of
the existing signal phasing, and signal coordination with the adjacent
intersections. Through this analysis we were able to identify the advantages
and disadvantages associated with each of the mitigating alternatives at
this location.
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