Problem 4:
Actuated Control
Printable Version
In our previous
analysis of the proposed signal at U.S. 95/StynerLauder Avenue, we have
considered the signal to be fixed time. In reality, most new traffic signals
that are installed today are actuated, responding to changing traffic
demands during the day. The HCM provides a mechanism to estimate green
times for an actuated controller, based on the relative traffic volumes on
each intersection approach, and to estimate the delay and level of service
that would result from traffic actuated timing plans.
Trafficactuated
control will generally accommodate a given volume of traffic with lower
delays than pretimed control, because of its ability to adapt to demand
variations.
The effects of
actuated control are reflected in the HCM analysis procedure in two ways:

The
equivalent cycle length and green times produced by the Appendix B procedure
are typically lower than their pretimed counterparts, yielding a lower
computed value of uniform delay.

As
illustrated in Exhibit 1613 of the HCM 2000, the incremental delay factor,
K, is given a lower value for trafficactuated control, depending on the
unit extension time and the v/c ratio. Lower K values also produce lower
delay estimates.
As we discovered in subproblem 2d,
we must also be aware of whether the intersection is part of a coordinated
system of intersections, where a fixed cycle length must be used. The
constraint of a fixed cycle length can have a significant effect on the
operating characteristics of an actuated controller. We will explore this
further in subproblems 4c and 4d.
[ Back
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with Problem 4 
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Problem 4:
Actuated Control
The trafficactuated
timing procedure requires three control parameters:

Minimum
phase times, considering driver expectancy and pedestrian requirements. The minimum times from the pretimed example (subproblem
1b) will be used here. 

Maximum
phase times to assure a reasonable distribution of green time on cycles with
heavy demand. The literature contains a variety of techniques for setting
maximum green times. For purposes of this example, the maximum green
times will be set to their corresponding pretimed values. 
 Unit
extension times to determine the length of the gap in arriving traffic at
which a phase will terminate. Most traffic models, including the HCM, will
yield lower delay estimates with lower unit extension times. As a
practical constraint, however, the unit extension must be slightly longer
than the maximum expected gap between vehicles departing from a queue, or
premature phase terminations will occur. For the purposes of this example, the
unit extension times will be set to 3 seconds for single lane operation and
2 seconds for multiplelane operation. 
This
problem illustrates some of the important elements of performing an analysis
of a signalized intersection operating under actuated control by addressing
the following issues as they relate to the proposed signal at the U.S. 95/StynerLauder Avenue intersection:
Subproblem 4a: Estimating phase times
for actuated signal control
Subproblem 4b: Effects of Unit
Extension on intersection operating characteristics
Subproblem 4c: Coordinating a system of
actuated controllers
Subproblem 4d: Coordinated operation of
an actuated controller with leftturn protection
[ Back ] [ Continue ] to SubProblem 4a 
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Subproblem 4a: Pretimed
Control vs.
Actuated Control
Whenever signalized control is considered for an
intersection, it is usually worthwhile to spend at least some time thinking
about and evaluating the different types of signal control that are
available for implementation. Signalized intersection analyses described
earlier in this case study have assumed the signal will operate in a
pretimed mode, but most new signals today also have the ability to operate in an
actuated mode. Typically, the kinds of questions that the analyst would want
to answer before deciding between pretimed and actuated control would
include the following:
Discussion:
Take a few minutes to consider these questions. When you are ready to
continue, click continue below to proceed. [ Back ] to Problem 4 [ Continue ]
with SubProblem 4a 
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Subproblem 4a: Pretimed
Control vs.
Actuated Control
In this exercise, we will
convert the existing pretimed example from subproblem 1b (see
Exhibit 111) to
a traffic actuated control to illustrate the differences between the timing
and performance measures from the HCM computations. Using Appendix B, Chapter
16 of the HCM with the control parameters set as discussed above, we
can estimate the average green times if the signal controller is actuated. Exhibit
138 shows the results of this estimation.
Exhibit 138. Estimated Average Phase
Times (sec) at StynerLauder

Movement 
Minumum Phase Time
(sec) 
Unit Extension (sec) 
Pretimed (SubProblem
1b) Also the max phase time for trafficactuated control 
Traffic Actuated
Control (By HCM Chapter 16 Appendix B) 
EastWest 
20 
3 
20 
15 
NorthSouth 
14 
2 
40 
16 
Cycle Length (sec): 
60 
31 
Note that the
estimated average phase times for trafficactuated control are shorter than
pretimed control. Perhaps even more surprising is the fact that the
estimated average phase time for the eastwest movement is actually below
the minimum phase time, which has been set to reflect the crossing time
requirements of pedestrians. This does not reflect an
unsafe situation because these are equivalent phase times for purposes of
computing delay. They reflect the fact that, because of low vehicular
volumes, the eastwest phases will sometimes not be displayed. When the
eastwest phase is displayed, it will always be at least 20 seconds
long (i.e., the minimum phase time). When it is not displayed, it
will obviously be zero seconds long. In this particular case, the average of
these various phase lengths is expected to be about 15 seconds under
actuated control. The very
short cycle (32 sec) is also an equivalent value, and should not be viewed
in the pretimed sense.
Because the eastwest phase is sometimes
skipped, the average lost time associated with this phase will also be less, during the
analysis time period, than would be the case if it were pretimed. Specifically, the average lost time for the eastwest phase is
equal to the average lost time that would occur if the phase were pretimed,
multiplied by the proportion of cycles during the analysis period when the
eastwest phase is NOT skipped.
The green times shown
in the table above for actuated control will
now be used in the estimation of the control delay and level of service for
the intersection.
[ Back ] [
Continue
]
with SubProblem 4a 
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Subproblem 4a: Pretimed
Control vs.
Actuated Control
Exhibit 139 summarizes the level of service and delay at each approach for a pretimed and actuated control.
Exhibit
139. Comparison of Pretimed and
TrafficActuated Operation (Datasets 2, 18) 
Approach 
EB 
WB 
NB 
SB 
Overall 
Pretimed delay (sec/veh) 
20.3 
22.0 
6.1 
6.5 
10.9 
Actuated delay (sec/veh) 
7.9 
8.3 
7.4 
7.9 
7.8 
Pretimed LOS 
C 
C 
A 
A 
B 
Actuated LOS 
A 
A 
A 
A 
A 
Note that trafficactuated control has produced a
substantial reduction in delay for eastwest traffic accompanied by a very
slight increase for northsouth traffic. Note also that the delays are
now more or less equal on all approaches, whereas the distribution of times
under the pretimed design favored the northsouth (arterial) movements at
the expense of the eastwest (cross street) movements. In apportioning
the times, the controller has clearly ignored the much longer maximum green
times for the northsouth phase. In other words, the northsouth phase
would never reach the maximum time, because the unit extension settings would
cause the phase to terminate whenever the queue of vehicles on the approach
has been serviced.
While the timing plan presented above produces lower
delays than the previous pretimed plan, the local agency may still wish to
favor the north/south movements because of the heavier movement on U.S. 95. The results of this exercise suggest that increasing the maximum green time
would not accomplish this goal. The maximum green setting will have a
definite effect on the green time distribution under high traffic volumes;
but when demand is low, the minimum green time is the only parameter that
will force a green time distribution that is not in keeping with the
distribution of demand.
[ Back ] [ Continue ] to SubProblem 4b 
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Subproblem 4b: Effect
of Unit Extension
In this exercise, we will consider the effect of the
unit
extension variable on the overall operating characteristics of an
intersection controlled in an actuated mode.
Consider again the StynerLauder/U.S. 95 intersection that
is at the center of this case study. If you were going to evaluate the
operational effects of an actuated signal controller at this intersection,
it would be very important to select an appropriate unit extension for
the purposes of the analysis.
This subproblem will help you address the following
issues, each of which is important to understand as you settle upon an
appropriate value for the unit extension variable:

What effect does a longer unit extension have on an
approach? 

Would a longer unit extension increase or decrease overall
delay on the minor street movement? The major street movement? 

Would a longer unit extension increase or decrease the overall
intersection delay? 
 How would the arrival type affect your decision to
implement a relatively long unit extension? 
Discussion:
Take a few minutes to
consider these questions. When you are ready to continue, click
continue below to proceed. [ Back ] [ Continue ] with SubProblem 4b 
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Subproblem 4b: Effect of Unit
Extension
Remember that, at the outset of this problem, we assumed
that the unit
extension times would be set to 3 seconds for single lane operation and 2
seconds for multiplelane operation. These are
commonlyused values for the unit extension and are consistent with an
expected saturation flow rate of about 1,900 vehicles per lane per hour of
green. Under these conditions, the 2/3second unit extension values can be
expected to have the effect of extending the green phase beyond its minimum
length only so long as vehicle spacing is not much greater than one would
expect under saturation flow rate conditions.
As a further illustration of the effect of the traffic
actuated settings, let us examine what would happen to the operation of the
StynerLauder/U.S. 95 intersection if we increased the unit extension times
for all phases to 5 seconds each. This would mean that a phase would not terminate until a fivesecond gap was
observed between vehicles on any approach. In other words, the controller
would be waiting for stragglers instead of passing control to the next phase
after the queue of vehicles has been serviced.
Exhibit 140 summarizes the results of this exercise. Using the same Appendix B
procedures that we used in subproblem 4a, we can see that the average phase times
have increased from 15 and 16 seconds to 19 and 24 seconds, respectively.
The cycle length has increased from 31 seconds to 43 seconds. It is also
interesting to note that the apportionment of green time now favors the
arterial movement to a greater extent, because the longer northsouth
maximum green time has allowed this phase to be extended more than the
eastwest phase. This is also evident in the proportionately greater
increase in the eastwest delays that are now being predicted.
Exhibit 140. Impact of Increased Unit Extension Times
on StynerLauder/U.S. 95 Intersection 
Movement 
Short Unit Extensions
(Subproblem 4a)

Long Unit Extensions
(5 sec each)

Phase Time 
Delay 
Phase Time 
Delay 
Eastbound 
15 
7.9 
19 
10.8 
Westbound 
15 
8.3 
19 
11.3 
Northbound 
16 
7.4 
24 
7.6 
Southbound 
16 
7.9 
24 
8.1 
Intersection Cycle 
31 
7.8 
43 
8.9 
Based on the information shown in
Exhibit 140, is it better to use the originallyestimated unit extension
values of 23 seconds, or the 5second unit extension value assumed for this
subproblem? The answer depends on what the analyst is trying to accomplish and the
environment in which the signalized intersection is located. In the case of
the StynerLauder/U.S. 95 intersection, it might be appropriate to use the
longer unit extensions on the northbound and southbound approaches of the
state highway in order to favor and benefit through traffic. The shorter
unit extensions might be appropriate to use on the eastbound and westbound
side street approaches in order to minimize overall intersection delay and
cycle length. An alternative approach, however, could also be used with
equal validity.
[ Back ] [ Continue
] to SubProblem 4c 
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Subproblem 4c: Coordinated Operation
With Actuated Control
The StynerLauder/U.S. 95 intersection is located within a
section of U.S. 95 that has the potential to be operated as a coordinated
system of signalized intersections. If the decision is made to signalized
the StynerLauder/U.S. 95 intersection, it will need to be designed and
operated in a way that supports and enhances any coordinated
signal system that may ultimately be implemented.
The matter of arterial signal system coordination is
complicated somewhat when one or more of the system signals operates under
actuated control. This subproblem will introduce you to the key issues to consider to appropriately evaluate the effects that
actuated control of the StynerLauder/U.S. 95 intersection will have on the
overall arterial signal system. Specifically, the following are key
considerations that would need to be taken into account:

How will pedestrian crossing times impact the minor
street movement phase? 
 How would an actuated signal be coordinated as part of
a network? 
Discussion:
Take a few minutes to
consider these questions. When you are ready to continue, click
continue below to proceed. [ Back ] [ Continue ] with SubProblem 4c 
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Subproblem 4c:
Coordinated Operation With Actuated Control
In the first two subproblems involving trafficactuated control, we have
assumed that the controller at the StynerLauder/U.S. 95 intersection will operate in an
isolated mode, independent of any other intersections on U.S. 95. It is,
however, more likely that the local agency would want to establish a
coordinated system of actuated controllers on this route to take advantage
of the improvements that could be derived from the progressive movement of
traffic on U.S. 95. This would require a full timing plan design, including
cycle length, phase split times, and offsets. Coordinated arterial system timing
design is beyond the scope of the HCM arterial analysis procedure. It would
therefore be necessary to use one of the several available arterial signal
timing software products for this purpose.
We will consider this example in the context of a
coordinated system operating on a 90 second cycle as was introduced in
Problem 2d (coordination effects of a new signal). Coordinated trafficactuated control systems are generally modeled as pretimed systems for purposes of timing plan design. This is because actuated controllers within a
coordinated environment are forced to operate under a constant cycle length. As a coarse
approximation of their internal logic, traffic actuated controllers are
usually represented as devices that will assign enough time to the cross
street to maintain a reasonable degree of saturation, with the remaining
time given to the arterial movements. If we follow that logic here,
we must first determine the maximum amount of green time that the side
street approaches can be expected to require. The maximum green time must be
at least long enough to accommodate pedestrian crossing time requirements,
which we have previously determined to be 20 seconds. If approaching traffic
volumes are very high on the side street, then the maximum green time may
need to be even longer to assure vehicle needs are also being
met. Applying
the HCM procedure under an initial presumption that pedestrian needs will
control side street maximum green times (i.e., a maximum green+yellow+allred
time of 20 seconds in this case), we find that the v/c ratio for the most
critical movement (WB through and right) is 71% for a 90second cycle. Thus, vehicle
needs are also being adequately met, so we can conclude that a timing
apportionment of 70 seconds to northsouth traffic and 20 seconds to east
west traffic would be an appropriate estimate of the average green+yellow+allred times for
a coordinated trafficactuated system. This apportionment would be
implemented by coordination hardware that would impose a background cycle of
90 seconds.
The presumption of a 70sec/20sec split of time is adequate for
conducting an operational analysis. In reality, however, the eastwest phase would be trafficactuated with a maximum
phase time somewhat longer than the 20 second pedestrian requirement to
provide for the occasional cycles that inevitably occur with heavierthannormal cross street demand. [ Back ] [ Continue ] to SubProblem
4d 
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Subproblem 4d: Coordinated
Operation With LeftTurn Protection
An issue that
commonly arises when actuated controllers are used within a coordinated
system is whether or not to provide
protected phasing to the major street
leftturn movements. This issue is appropriate to
address for the proposed new signal at the StynerLauder intersection.
In subproblem 4d, we will demonstrate how to address this
question at the U.S.
95/StynerLauder Avenue intersection. Here are some issues to consider as you
prepare to conduct this analysis:

Do existing or projected volumes warrant protected
leftturn phasing at this intersection? 

How is the green time allocated between the through and
leftturning phases on the major street? 
 What are the delay implications of providing protected
leftturn phasing on the major street? 
Discussion:
Take a few minutes to consider these questions.
Click continue when you are ready to proceed. [ Back ] [ Continue ] with SubProblem 4d 
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Subproblem 4d: Coordinated
Operation With LeftTurn Protection
The left turning volumes are very light at this intersection, and all of the
previous problems have assumed a simple twophase operation. On the other
hand, some agencies prefer to provide left turn protection even when leftturning volumes are low with the idea that the leftturn
phases will be skipped if there is no demand during a given cycle.
We will therefore consider the addition of protected
phases for the arterial (northsouth) left turns, retaining a 90second
cycle. We will keep the same settings for the cross street through phases
and add a phase for the northbound and southbound left turns with a 12second
maximum phase time consisting of 8 seconds green and 4 seconds
yellow plus all red. The minimum green will be 8 seconds, so the full 12second phase time will be displayed on cycles during which any leftturning
vehicles arrive on the approach served by the leftturn phase.
In summary, a pretimed representation of the proposed
signal timing would be 12
seconds for NB and SB left turns, 58 seconds for NB and SB through, and 20
seconds for all EB and WB movements. Since the left turns are very light,
the phase will be skipped on some cycles and the unused time will be added
to the through movements. The objective of this exercise is to
determine how much green time on an average cycle will be added to the
arterial through traffic phase. The amount of added green time must be based
on the probability of one or more leftturn arrivals during any given cycle.
[ Back ] [ Continue
] with SubProblem 4d 
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Subproblem 4d: Coordinated
Operation With LeftTurn Protection
In order to estimate the proportion of skipped leftturn
phases during the analysis time period, we need to estimate the probability
that no leftturn vehicles will arrive during a cycle. For this purpose, it is common to assume that the
leftturning vehicles will arrive at the intersection according to a Poisson
distribution (a Poisson distribution reflects random
arrivals). Assuming a Poisson distribution of arrivals, the
probability of zero arrivals on any cycle may be computed as:
where a represents the average number of arrivals per
cycle, based on the hourly volume.
Exhibit 141 shows the results of these computations. If the average length of a leftturn phase is less than 12 seconds, the
difference is added to the phase time for the opposing through movement.
As an example, we will examine the average northbound
phase times. The average cycle length at this intersection is 90seconds, or
40 cycles per hour. The northbound leftturn volume is thirtyone vehicles
per hour, which produces an average of 0.78 vehicles per cycle (31 vehicles
per hour/40 cycles per hour). From the above equation we estimate zero
arrivals on 46percent of the cycles (e^{0.78}), giving us
an average phase time of 6.5 seconds (12 sec * (10.46)). Because this is
less than the assumed maximum of 12seconds green, we may add the difference
in time (5.5 seconds) to the conflicting southbound through phase, producing
63.5 seconds of green (58 sec+5.5 sec).
Exhibit 141. Estimation of the
likelihood of zero leftturning arrivals 
Left Turn Properties 
Average Phase Time (sec) 
Direction 
Volume (vph) 
Arrivals
per cycle 
Probability of zero arrivals 
Left 
Through 
NB 
31 
0.78 
0.46 
6.5 
60.7 
SB 
59 
1.48 
0.23 
9.3 
63.5 
The delay computations may use the average phase times for the through and leftturn
movements determined from the table shown above.
[ Back ] [ Continue
]
to Problem 4 Analysis 
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Problem 4: Analysis
In Problem 4, we
explored the effects of trafficactuated control on the performance of the
U.S. 95/StynerLauder Intersection. Trafficactuated control is a function of
the detection system and the signal timing active within the signal
controller at the intersection. In detailed operations analysis, the use of
detector timing presents an additional level of complexity for our analyses
and thus, may more closely reflect field conditions.
In subproblem 4a,
we considered the effect of actuated control that revisits our assumption
regarding cycle length and phase time. This analysis assumes fullyactuated
control typical of an isolated operations.
In subproblem 4b,
we examined the effect of unit extension on phase length and its
corresponding effect on cycle length at a fullyactuated intersection. We
learned that as the unit extension (commonly referred to as passage gap in
signal controller manuals) increases, the phase length and cycle length
increase.
In subproblem 4c,
we revealed the implications of a coordinated operations on an actuated
signal. Specifically, the introduction of a fixed cycle length may result in
increased delay to the side street but improved operations and progression
along the arterial.
Finally, in subproblem 4d,
the effect of leftturn protection is considered. In this analysis, we learn
how to estimate the percentage of cycles that will result in leftturn
actuation.
[ Back ] [ Continue ] with Analysis 
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Break
Problem 4: Analysis
The trafficactuated
timing procedure requires three control parameters.
 Minimum
phase times, considering driver expectancy and pedestrian requirements. The
minimum times from the pretimed example (Subproblem 1b)
were used here.
 Maximum
phase times to assure a reasonable distribution of green time on cycles with
heavy demand. The literature contains a variety of techniques for setting
maximum green times. For purposes of this example, the maximum green times
were set at 20% longer than the pretimed values.

Unit
extension times to determine the length of the gap in arriving traffic at
which a phase will terminate. Most traffic models, including the HCM, will
yield lower delay estimates with lower unit extension times. As a practical
constraint, however, the unit extension must be slightly longer than the
maximum expected gap between vehicles departing from a queue, otherwise
premature phase terminations will occur. For purposes of this example, the
unit extension times were set to 3 seconds for singlelane operation and
2 seconds for multiplelane operation.
[ Back ] [ Continue ] to Problem 5

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Break
Problem 4: Discussion
The decision to
signalize an intersection brings with it many additional issues that also
need to be addressed. Should the signal operate in a pretimed,
semiactuated, or fullyactuated mode? Should it be coordinated with
adjacent signalized intersections or should it be designed to operate as
an isolated intersection? How should the leftturns be handled on each
approach  should they be permitted, protected, or both? What should be
the phasing sequence? What are the appropriate settings for the various
signal timing parameters, including in the case of actuated control the
factors of minimum green time, maximum green time, and unit extension? The
answer to each of these questions is affected by the answers to all the
others, and so it is typical that multiple scenarios will need to be
investigated before deciding upon a particular implementation plan.
A fairly
comprehensive method is presented in Chapter 16, Appendix B of the HCM2000
for reasonably approximating trafficactuated operation. This method takes
explicit account of many phasing variables, detector design parameters,
and controller settings. Its application in Problem 4 provided
considerable insight into the relative merits of pretimed versus actuated
signal control at the U.S. 95/StynerLauder intersection.
The next problem
steps away from the locale of the U.S. 95/StynerLauder intersection and
recognizes that the U.S. 95 facility includes both urban and rural
segments. In particular, Problem 5 focuses on a rural segment of U.S. 95
located south of Moscow, and identifies a type of twolane highway
analysis that is not explicitly addressed within the current version of
the HCM.
[ Back ] [
Continue
] to Problem 5 
