Traffic signals do not prevent crashes. Engineering studies have shown that in many instances, total intersection crashes increase after a traffic signal is installed. Certain types of crashes are susceptible to correction by installation of traffic signals, however, overall the number of crashes increase.
When determining whether or not a traffic signal is necessary at a specific location, an evaluation of the candidate location (called a signal warrant study) is conducted to determine the answers to the following questions:
- How much traffic is there on the intersecting streets?
- Are high levels of traffic consistent throughout the day or just during a few hours?
- Is there a lot of pedestrian traffic?
- Is the street a wide, high speed and busy thoroughfare?
- Are school children crossing the street?
- Will a signal improve the flow of traffic or cause gridlock with other nearby signals?
The signal warrant study collects all of the relevant data at a location that is being considered for a traffic signal. Once the data are collected, they are compared to standards established by extensive research and experience and documented in the latest edition of the Manual on Uniform Traffic Control Devices. These standards, called "Traffic Signal Warrants" are used by traffic engineers throughout the United States to help determine appropriate signal locations.
A properly placed signal can improve the safety and efficiency of flow through an intersection. An unnecessary signal can be the source of danger and annoyance to all who use the intersection including pedestrians, bicyclists and motorists. It can also increase air pollution and cause driver frustration if there is not much traffic on the major street.
Satisfaction of the signal warrants do not necessarily justify installation of a traffic signal. Other, more appropriate solutions should be considered prior to considering installation of a traffic signal. Spacing between signals is always a major concern beyond the basic warranting analysis and should be carefully reviewed before deciding on installation of a new signal.