FHA Signal Timing On A Shoestring

IV. Local Controller Parameters
It is useful to identify traffic signal timing parameters as being in one of two categories:
local intersection timing parameters and coordinated operation timing parameters. The
phase minimum or yellow change times are examples of local intersection timing
parameters. The intersection cycle length and offset are examples of coordination timing
parameters. There are far more local intersection timing parameters than coordination
timing parameters. In fact, most systems define only three coordination timing
parameters: cycle length, offset, and split. The following section of this report describe
methods that can be used to set the local intersection timing parameters while the next
section deals with the coordinated operation parameters.
The basic timing parameters are essentially the same for all actuated controllers. There
are subtle differences between different software implementations, for example, the NEMA
controllers define the Force-off function as a “per ring” function while other
implementations define the Force-off function as a “per phase” function. This distinction
has little importance to the Traffic Engineer who is responsible for developing new Traffic
Signal Plans. These differences, however, are very important when the results of a signal
timing optimization process are implemented in a particular controller. Many newer
controllers automatically calculate the force-off setting once the engineer defines the split.
Because most jurisdictions use NEMA TS-1 and TS-2 or Model 170 actuated controllers, the
primary emphasis has been placed on timing actuated controllers. It is important to
recognize, however, that many jurisdictions use Pre-timed controllers. Most of the
principles noted in this section can be directly applied to Pre-timed controllers.
For the current status on signal timing, the reader is directed to Signal Timing Practices
and Procedures: State of the Practice (Item IR-116) published by the Institute of
Transportation Engineers. This report documents the current state-of-the-practice in
traffic signal timing. It was prepared as part of a continuing program developed by FHWA
and ITE to place increased emphasis on the quality of U.S. traffic signal timing.
Actuated Controller Timing Principles
Some of the basic principles of timing the green interval in a traffic actuated controller unit
are as follows:
Each phase must have a minimum green time so that a stopped vehicle that receives
a green signal has enough time to get started and partially cross the intersection
before the yellow signal appears. This time is sometimes called the initial portion of
the green interval.
Each vehicle requires enough green time to travel from the detector to the
intersection. This is called passage time, vehicle extension, or gap. Gap refers to
the distance between vehicles as well as the time between vehicles. Each successive
vehicle actuation, therefore, increases the phase green time. With no opposing calls
the controller rests. Extensions continue being timed, but with no effect on the
green interval.
There must be a maximum time that the green interval can be extended if opposing
cars are waiting; this is called the Maximum Green or extension limit.
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