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Chapter 4: Actuated Traffic Controller Timing Processes
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In Chapter 4, you will learn about the timing processes that run an actuated traffic controller. Many
transportation engineers begin their study of signalized intersections by making assumptions on the length
of the cycle and the duration of the green intervals for each phase. However, the reality is the cycle length
and the green interval durations are the result of the interaction of the traffic demand and a set of actuated
controller timing processes. If the demand is low, the phase may only last until the minimum green timer
has expired; but if the demand is high, the phase may be extended (by a series of intervals equal to the
vehicle extension time for each vehicle that arrives at the intersection) until the maximum green time has
been reached. Thus the green duration and the cycle length will vary as the demand varies from one cycle
to the next.
The Highway Capacity Manual or HCM (Transportation Research Board, 2010) includes a method for
synthesizing the green interval duration based on demand and some of the timing parameters described in
this section. However, in order for you to develop a signal timing plan, it is important for you to understand
these actuated controller timing parameters and the process followed by each.
When you have completed the activities in this chapter, you will be able to
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Describe the actuated controller timing processes
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Complete a traffic control process diagram describing the response of the detectors, the timing
processes, and the displays to a pattern of vehicle demand
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Describe the range of information provided in the ASC/3 controller
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Describe the effect of detector calls on controller timing processes
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Describe the two primary methods of terminating a traffic phase at an isolated intersection
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Describe the actuated controller timing processes
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Infer signal timing parameter values through field observations
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Contrast the signal timing terms that are presented in this chapter with those described in the
Traffic
Signal Timing Manual
This chapter begins with a
Reading
(Activity #17). The reading describes and illustrates the three basic
actuated controller timing processes, establishing your initial knowledge base of these processes. This
reading covers the minimum green time, the passage time, and the maximum green time. Each of these
three is a timing parameter, the value of a timing parameter, and a timing process. In Activity #18, you will
assess your understanding of the material presented in the
Reading
by constructing three traffic control
process diagrams. In Activity #19, you will study the ASC/3 traffic controller to learn about how basic
actuated timing processes operate and are displayed in the controller front panel. In Activity #20, you will
use a side-by-side movie of a VISSIM simulation to observe the two different ways in which a phase first
times and then terminates. Activity #21 leads you through additional studies of the traffic controller and
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Actuated Traffic Controller Timing Processes