Do Traffic Signals Reduce Accidents At Intersections?
Traffic signals don't always eliminate collisions.
In fact, in most instances, collisions and severe injuries increase after signals are installed. The Manual on Uniform Traffic Control Devices suggests that signals should not be installed under the premise of reducing collisions, unless five or more occur per year. This was predicated on a comprehensive California study which determined that of all signals installed over a five year period, the only ones that enjoyed a reduction in collisions were those that had at least five occurring annually beforehand. Another study, the Schoene Study, indicated that an increase in collisions should be expected at intersections having fewer than ten right-angle collisions per year prior to signalization.
The purpose of signals is to improve the overall flow of traffic. Signals can be helpful in reducing right-angle collisions under certain circumstances, but almost always increase other type of collisions such as rear-end and left turn collisions. Unfortunately, signals can also lull pedestrians into a false sense of security.
Traffic engineers compare existing conditions against guidelines established after many years of study throughout the country.
If conditions do not meet established guidelines, signalization will only cause more collisions, congestion and delay. When these conditions are met, an engineer then needs to consider geometric conditions and spacing to ascertain whether or not a signal could be helpful.
Signals installed at collector and local street intersections may be necessary to improve access to major streets, but they can result in more neighborhood cut-through traffic.
Traffic engineers consider these points when deciding if a traffic signal will help more that it will hurt:
Is congestion severe enough to create extraordinary frustration?
Is traffic so heavy that drivers on the side street try to cross when it is unsafe?
Is there a large number of pedestrians waiting to cross a wide, high speed and busy street?
Does the age and number of school children crossing the street require special controls? If so, is a traffic signal the best solution?
Would a signal allow for a smooth flow of traffic and not adversely affect safety by causing gridlock with a nearby signal?
A traffic safety message from the Phoenix Street Transportation Department.
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