Printable Version
Problem 3: Signal Timing Analysis of Rietz Union
Drive/Museum Road
In Problem 2 we explored signalization of the Rietz Union
Drive/Museum Road intersection with development of the parking garage,
including various phasing plans and cycle length mitigations to reduce both
vehicular and pedestrian delay.
Several assumptions were made regarding the signal timing
parameters at the intersection which simplified our analysis. In this
subproblem, we will further explore those assumptions and the effect that
changes to these parameters have on the intersection performance.
We will also explore the extent that a local jurisdiction's
operational requirements have on the actual performance of an
intersection. In this instance, we will explore the effect of various signal
timing parameters on volume to capacity ratio and how this relates to signal
timing within the signal controller.
In this problem, consider the following issues as
you work through the computations for the three subproblems:
 What is the effect of coordinated operations at an
intersection? 
 What does a corridor with a low volume to capacity
ratio look like in the field and how does various detector timing
parameters affect the operations? What does this look like in the field? 
 What are the various ways to optimally distribute green
time at an intersection? 
 What is the effect on other users of the signalized
intersection (pedestrians, transit, etc)? 
This problem illustrates some of the important elements
regarding the differences between HCM operations analysis methodologies and
results and field operations. The problem is separated into three
subproblems:
Subproblem 3a: Actuated Signal Detection and Signal
Timing
Subproblem 3b: Software Analysis Techniques and Implications of Calculation
Techniques on v/c ratio
Subproblem 3c: Effects of Pedestrians on Signal Timing, Pedestrian Delay
Discussion:
Consider
what it means for a movement to operate at a volume to capacity ratio of
0.80 in terms of intersection efficiency. What might a volume to capacity
ratio of 0.80 look like in the field? Consider this question. When you are ready to continue, click the
link below to proceed.
[
Back ] to Getting Started [
Continue ] to Subproblem 3a 
Page Break
Subproblem 3a: Actuated Signal Detection and Signal
Timing
Step 1. Setup
In subproblem 3a, we will evaluate the operational
characteristics of the signalized intersection and how the coordinated
signal timing parameters affect operations in the field, considering the
effect of various volume to capacity ratios on performance. Before we look at the
volume to capacity ratios, we will
first discuss how actuated signals interact with detectors to determine
phase lengths. As mentioned in previous subproblems, there are three basic
types of signal control: actuated, semiactuated, and pretimed. In this
subproblem we will explore actuated signal control in coordination as
it relates to the Reitz Union Drive/Museum Road intersection.
Because Museum Road is a coordinated corridor, a fixed
cycle length is required to maintain corridor progression. In Problem 2
we assumed the cycle length for the corridor was 120 seconds and that each
approach was phased permitted. In this
subproblem, we will explore how this assumed cycle length is broken into
phase lengths. Not only will we look at the average for the hour, but we
will also look at how
individual cycle lengths vary throughout the hour.
Discussion:
Why
would a corridor with progression need to maintain a fixed cycle length?
What is the difference between looking at an average hourly cycle versus
individual cycles? How do signal timing parameters affect the phase
length? Take
a few minutes to consider these questions. Click continue when you are ready
to proceed.
[
Back ] to Problem 3 [
Continue ] with Subproblem 3a 
Page Break
Subproblem 3a: Actuated Signal Detection and Signal
Timing
Let's discuss each of the questions
posed on the last page.
Why would a corridor with progression
need to maintain a fixed cycle length? A fixed cycle length is
frequently used to maintain coordination between adjacent intersections
where the traffic engineer seeks to preserve the quality of progression. The
arrival type is the primary input to the methodology that affects this
calculation.
A cycle length is imposed upon an
intersection by assigning a coordinated movement or group of movements to
the intersection. This is typically the main street through phase. Once this
is established, other phases at the intersection are actuated, meaning that
the time allotted to each of these phases is dependent upon the signal
timing parameters and demand, or as the HCM states: these phases "can be
extended by reassigning unused green time from actuated phases with low
demand." In our case, Museum Road is the coordinated movement and Reitz
Union is actuated. Thus, if Reitz Union does not have any demand during a
cycle, Museum Road will stay green throughout the cycle. Essentially,
whatever time is unused on Reitz Union is received by Museum Road because
the traffic controller will maintain progression by giving remaining green
to the coordinated phase.
What is the difference between
looking at an average hourly cycle versus individual cycles? The difference between
looking at average phase lengths and phase lengths throughout the hour
depends on the variability of traffic volumes. Volumes vary cycle by cycle throughout the hour.
Exhibit 5X shows the phase length versus the number of occurrences.
What should be noted from this figure is
that throughout the hour the phase length varies as arrivals vary.
The Museum Road phase length would show an inverse relationship due to the effects
of coordination within the controller, since it receives the additional
time. In other words, in the cases where the phase length is 45 seconds for
Reitz Union (40 seconds of green time), Museum Road is 70 seconds of green,
and when Reitz Union is 20 seconds long, Museum Road is 90 seconds.
Before we can answer this, we will take
a closer look at the signal timing parameters.
[
Back ] [ Continue ] with Subproblem 3a

Page Break
Page Break
Subproblem 3a: Actuated Signal Detection and Signal
Timing
How do signal timing parameters affect the phase
length? As we have seen from the previous page, the green time for
individual phases with actuated control is variable. The time assigned to
each phase is governed by three parameters, the minimum green setting, the
maximum green setting, and the unit extension, also known as the passage
gap.
 The minimum green setting defines the minimum amount of
time allocated to the green phase assuming no vehicles are detected that
would extend the phase. This does not include yellow or all red time. 
 The maximum green setting defines the amount of time after
which the phase will be terminated, regardless of the unit extension. 
 The unit extension, or passage gap, defines the amount of
time between vehicles at which the phase will be terminated before the
maximum green time. The unit extension will not terminate the phase before
the minimum green time. 
The time allotted to each phase is dependent on the demand
at the intersection and these signal timing parameters. In the case where
demand is light, the remaining time from each phase is reallocated to the
coordinated phase. Pedestrians also have an influence on phase lengths, but
for simplicity, within this first subproblem we will ignore pedestrian
effects.
Two other factors may change the passage gap of a phase: TBR,
or time before reduction, and TTR, time to reduce the passage gap as the
phase extends past the initial value of unit extension
time. These factors help to catch the platoon as it moves along a corridor.
The TBR defines the amount of time the unit extension is held steady at its
higher initial value. After the TBR, the unit
extension is reduced, since at this point the queue that was waiting for the
light is moving at a steady speed across the detectors. The unit extension
value is reduced in a linear manner to the minimum gap time. The time that
passes from the TBR to when the unit extension is at the minimum gap time is
the TTR. Exhibit 5X below
demonstrates this concept, with an inital passage gap of 2.5 s, a minimum
gap of 1.0s, a TBR of 12s, and a TBR of 15s.
On the next page, we will apply this information to our
example.
[
Back ] [ Continue ] with
Subproblem 3a 
Page Break
Subproblem 3a: Actuated Signal Detection and Signal
Timing
If we look back at the analysis
completed in Subproblem 1b, page2, we assumed the default value of unit extension
(3.0 seconds) and a g/c ratio of 0.33 for Reitz Union. However, from the
discussion on the last two pages, we found that the phase length varies as a
function of the signal timing parameters and thus, depending on the
parameters the g/c ratio, may change accordingly. Essentially, the phase
length is a function of the unit extension; once traffic is served, the
controller times the gap setting as extra time looking for an additional
vehicle to cross the detector.
Step 2. Results
How do signal timing
parameters affect the phase length and the volume to capacity ratio in the
HCM methodology? Let's
assume our minimum green
time is 5 seconds, our maximum green time is 40 seconds (as was used in the
HCS analysis from Subproblem 1b, page2), and our unit
extension is three seconds. If we reduce the unit extension, we would expect
the actuated side street movement (Reitz Union) to receive less green
time. HCS calculates that a shorter unit extension reduces the delay for the
entire intersection because there is potentially less time wasted at the
intersections.
If we make this change in our
HCS file, now assuming that the unit extension is one second, (because we
have detection on the north and south approaches that allows shorter unit
extension time) (Reitz3a4  ask PJK) we find that changing the unit
extension does not change the g/c ratio assumed in the analysis for the
north/south approach. This is due to the procedures within HCS represent
analyses of fixed lengths of intervals and sequences of phases. In other
words, there is no direct link between the g/c ratio assumed and signal
timing parameters such as unit extension. The reduced unit extension does
result in a reduction in delay for the intersection and each individual
movement in the calculation.
Is this representative of
what happens in the field? This may not be representative of what occurs
in the field and for a detailed analysis, measured green time for each
approach may be used to refine the analysis. Reducing the unit
extension would reduce the g/c ratio for the north/south movement. The
coordinated phase, Museum Road through (east/west) would receive the
additional time that is unused from the Reitz Union approaches.
Report results from analysis here...
Discussion:
Take
a few minutes to consider how much more the phase times could vary if we
assumed equally spaced random arrivals at the intersection. What conditions would make
the 50thpercentile phase length the same as the average phase length?
How significant is this difference in phase length?
[
Back ] [ Continue ] with Subproblem 3a 
Page Break
Subproblem 3a: Actuated Signal Detection and Signal
Timing
Unlike with unsignalized intersections,
the volume to capacity ratio for a signalized intersection will vary
depending on the signal timing parameters. If an intersection approach operates
overcapacity, adjustments in the green allocation could be made to alter
the operational characteristics of the approach.
As previously mentioned, Museum Road
will be coordinated with development of the signal at Reitz Union Drive.
This will be necessary not only to decrease corridor delay, but, due to
their close spacing, also to prevent queues from extending through adjacent
intersections. Because we have a fixed cycle length, the intersection will
operate as if it were semiactuated, with additional time from the minor
street phases reassigned to the major through street phases.
The jurisdiction may also require the
intersection operate at a Level of Service "E" or better, asking that a
volume to capacity ratio less than 0.80 be maintained.
What does it mean for the minor
street approach to operate at a v/c ratio of 0.80? On Reitz Union, we
will assume that the queue accumulates during the red time, and that once the
phase is green, saturated flow will occur on the through movement.
During each phase, the volume to capacity ratio is 1.0 as long as saturation
flow is achieved and maintained. As the green per cycle ratio is increased
for an intersection, the v/c ratio is reduced assuming all other things being equal.
Thus, as time is provided to a particular movement that is unutilized, the
volume to capacity ratio is reduced.
As an example, let's assume we had
sixteen
vehicles in queue at the start of the green phase and based on our
saturation flow rate we assume the intersection would need 30 seconds for
queue clearance. Once the phase gapped out 3 seconds later at 33 seconds,
(due to a 3 second unit extension), the
approach would be operating at a v/c ratio of
0.91, adding 3 seconds of wasted delay to conflicting movements. If we were
to maintain a v/c ratio of 0.8 at this approach, this phase would be 30/0.8
seconds, or 37.5 seconds. This would equate to 7.5 seconds of wasted time being added to the delay of conflicting movements.
What this suggests is that there is a difference between the volume to
capacity ratio reported for planning purposes and the way these
intersections operate in the field. Analyses that require a future year
volume to capacity ratio of 0.80 may suggest a planning method that is
insensitive to detector operations being used.
Discussion:
Take
a few minutes to consider why the v/c ratio for side streets may be greater
than 1.0. If you were to hold the g/c ratio constant, what could you do to
reduce the v/c ratio for the side street? How would this affect delay?
[
Back ] [
Continue ] to Subproblem 3b 
Page Break
Subproblem 3b: Pedestrian Level of Service
Step 1. Setup
Throughout this case study we have looked at pedestrian
and vehicular mitigations at the Reitz Union Drive/Museum Road intersection.
In this subproblem, we will estimate pedestrian delay at the intersection
and look at the interaction between vehicles and pedestrians. In addition,
we will explore some of the factors that must be considered when analyzing
pedestrian delay.
Discussion:
Consider
what factors might affect pedestrian delay at a signalized intersection. How
could these factors be changed to reduce pedestrian delay? How would this
affect vehicular delay? Click continue when you are ready to proceed.
[
Back ] to Subproblem 3a [
Continue ] with Subproblem 3b 
Page Break
Subproblem 3b: Pedestrian Level of Service
Step 2. Results
There are two primary factors related to pedestrian delay
at a signalized intersection: cycle length and effective green time for
pedestrians. Intuitively, longer cycle length and less effective
green time for pedestrians results in higher pedestrian delay. In addition, factors
such as the effective sidewalk width and available storage area at each corner may
also affect pedestrian delay.
Near the university, where pedestrian volumes are high,
consideration should be given to reducing cycle times, possibly by looking
at doublecycle options as we explored in Problem 2. Shortening the cycle
time will reduce pedestrian delay, which in turn reduces pedestrian
noncompliance.
Using HCM Equation 185 and our proposed signal timing in
Subproblem 2c, the average eastbound pedestrian delay is just over
X
seconds, which suggests a pedestrian LOS
X with
low likelihood of pedestrian
noncompliance. Because this signal phasing uses an exclusive pedestrian
phase, the delay for northbound/southbound pedestrians is the same for
eastbound/westbound pedestrians.
Discussion:
What
is assumed when calculating pedestrian delay? How does this affect the
results?
[
Back ] [
Continue ] with Subproblem 3b 
Page Break
Subproblem 3b: Pedestrian Level of Service
In pedestrian delay calculations, how do you arrive at the effective green time for pedestrians? In general,
you would use
the effective green time for the concurrent traffic movement minus the
flashing don't walk time. The effective green time wouldn't be
the average effective green time, as we would want to consider only those phases where
there was a pedestrian call.
Why? Minor street green allocation is less
than that for the major street. When a pedestrian call is present,
additional green time needs to be assigned to the minor street approach to
accommodate pedestrians' longer crossing time across the wider, major
roadway. If we were to use an average phase time, we would include phases
where no pedestrian call was present, thereby reporting a lower minor street
effective green time. When we calculate pedestrian delay, we assume
that pedestrian compliance is 100%. This means that a pedestrian arriving
at the start of the flashing don't walk will wait until the next cycle, and
a pedestrian waiting while no conflicting traffic is present will also wait.
However, we also assume that all vehicles, especially in regards to channelized rightturn on reds, obey the rightofway and yield to
pedestrians. Pedestrian characteristics
may also affect pedestrian delay. At an intersection of two major roadways,
a pedestrian that hits both pushbuttons may have to longer than if they
were to hit the correct one. Further complicating the analysis, some
highvolume roadway intersections expect pedestrians to wait at a center
median before finishing their crossing.
These are some of the factors that must be considered when
analyzing pedestrian level of service. In most cases, the pedestrian delay
equation will be a worstcase analysis scenario that gives an upperbound to
pedestrian delay. However, under certain circumstances this may not be the
case. [
Back ] [ Continue ] to Subproblem
3c 
Page Break
Subproblem 3c:
Step 1. Setup
Discussion:
Take
a few minutes to consider these questions. Click continue when you are ready
to proceed.
[
Back ] [ Continue ] with
Subproblem 3c 
