Traffic signals are the red, yellow and green lights at busy intersections that direct approaching traffic to stop and, then, to proceed. At the proper location and under the appropriate conditions, a traffic signal can reduce traffic delays and enhance safety.
There is a widely-held belief that a traffic signal can solve all intersection traffic problems. But, under the wrong circumstances, a traffic signal can cause unnecessary delay, waste taxpayer dollars, and actually increase crashes. Simpler measures sometimes work better.
This introduction explains what a traffic signal can do, what it cannot do, how traffic engineers can improve safety and reduce delays at intersections, and what happens when it is determined that a traffic signal is the best solution.
A traffic signal sequentially assigns the right-of-way to vehicles approaching an intersection from various directions, forcing the streams of traffic to take turns entering the intersection. A signal stops traffic in one or more directions so that:
Most signals have vehicle detectors, usually imbedded in the roadway, that sense traffic approaching from each direction and adjust signal timing for maximum efficiency.
A signal can reduce the delay for those waiting to cross a heavy stream of traffic and reduce hazardous conflicts between traffic movements, thereby reducing the likelihood of collisions. At certain locations it can help establish a "signal progression," so traffic can continue along a route at a reasonable, constant speed and with minimal delay.
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