TRAFFIC SIGNALS. IN: HANDBOOK OF TRANSPORTATION ENGINEERING

This chapter describes how at the intersection of two streets with no traffic control, drivers are expected to decide for themselves who is to go and who is to wait. Their only guidelines are the rules of the road, as defined in the Uniform Vehicle Code and the laws of the state, and their courtesy and cooperation. For intersections where the traffic demand is very light and the conditions favorable, drivers do not have trouble assigning their own right-of-way in this manner, and their delay is very small. As soon as traffic begins to build, drivers are faced with too many decisions to make in order to safely assign their own right-of-way, and some help from the governing body is required to help them with these decisions. The simplest form of traffic control is a yield sign on one of the streets to let drivers know which movement has the right-of-way when a decision must be made. This, too, is only suitable for low traffic volumes. As traffic increases, drivers must be stopped so that they have time to make complex right-of-way decisions. A two-way stop, with stop signs on the minor street, serves this purpose. Eventually the traffic will build to the point that assignment of right-of-way is not the only problem, and moving the traffic efficiently becomes a primary concern. Stop sign are effective in the former task, but they force every driver to stop and start and this is inefficient in that a large proportion of the time is spent just waiting for the drivers to get going. At this point, traffic signals can assign right-of-way much more efficiently because the delay associated with starting up is only left by the first two or three cars in the platoon. The ideal isolated traffic signal assigns just enough green time to clear the queue of traffic and then move on to the next movement. When other signalized intersections are nearby, traffic engineers also try to arrange the timing so that the light will be green for platoons coming for the other signals when they arrive. These two goals sometimes compete. Current control equipment can provide a wide array of features, loosely organized into fixed-time and traffic-actuated capabilities. The former are usually used for calculating signal timings, especially in systems where those timings have to be consistent to maintain network flow, and actuated settings are used to allow the local controller to make adjustments based on the demand actually present at the intersection.