Goal of Signal Timing
The goal of signal timing and coordination is to get the greatest number of vehicles through a particular corridor and to improve the flow of traffic along a major street or through a network of streets.
Traffic signals assign the right-of-way to various traffic movements. Main Street is one of the most heavily traveled streets in the City of Woodland. Main Street, between Cleveland and Third Street, is operated as an interconnected pre-timed system. The signals have preset time intervals for different times of the day, including morning, noon, and evening peak travel periods. The intersections of Third Street through Cleveland Street along Main Street are interconnected by a seven-wire cable to ensure coordinated operation. Timing plans are normally selected by a time clock. If a signal should get out of step, the system automatically corrects itself. The local controller for one intersection acts as the master controller for the system.
Traffic actuated signals use detectors located in the pavement on the approaches to traffic signals to monitor and assign the right-of-way on the basis of changing traffic demand. These signals attempt to give most of the available green time to the heaviest traffic movements. The cost of an actuated signal is much greater than the cost of a pre-timed signal.
Actuated vs. Fixed Time Signal Control
Traffic signals are programmed to produce the best flow of traffic for each individual intersection. Furthermore, there are different methods in which traffic signals are programmed to run. For example, signals can be timed to operate on an actuated or a fixed time signal control. Fixed-timed signals, or pre-timed control, is a consistent and regularly repeated sequence of signal indications that is given to traffic. A good example of this type of signal control occurring in Woodland is along Main Street in the downtown core. Here, the pre-timed operation provides consistent interval timing from cycle to cycle under the given traffic pattern.
On the other hand, semi-actuated or full-actuated timed signals occur when the duration of some or all of the intervals vary from cycle to cycle; or some phases may be omitted altogether during a cycle. The duration of some or all of the intervals and whether a phase is served or omitted is determined based on actuations from detection equipment. A good example of a traffic signal in Woodland that is operating on an actuated-timed signal basis is East Main Street at Pioneer Avenue.
Coordinated vs. Independent Traffic Signal Operations
In addition to timing an individual traffic signal, some signals are also timed as coordinated in order to achieve a total network of unified traffic signals. This is a predetermined timing relationship among adjacent signals. The goals of coordinating the signals are to minimize delay, minimize stops, and to progress the flow of traffic.