Traffic signals don't always prevent accidents. In many instances, the total number of accidents and injuries increase after they're installed.

Where signals are used unnecessarily, the most common results are a reduction in right-angle collisions but an increase in total accidents, especially the rear-end type collision. In addition, pedestrians are often lulled into a false sense of security.

In deciding whether a traffic signal will be an asset and not a liability, traffic engineers evaluate the following criteria:

• Does the number of vehicles on intersecting streets create confusion or congestion?

• Is traffic on the main street so heavy that drivers on the side street will try to cross when it is unsafe?

• Does the number of pedestrians trying to cross a busy main street create confusion, congestion or hazardous conditions?

• Does the number of school children crossing a street require special controls for their protection? If so, is a traffic signal the best solution?

• Will the installation of a signal allow for continuous, uniform traffic flow with a minimum number of vehicle stops?

• Does an intersection's accident history indicate that a signal will reduce the possibility of a collision?

Traffic engineers compare the existing conditions against nationally accepted minimum standards established after many years of studies throughout the country. At intersections where standards have been met, the signals generally operate effectively with good public compliance. Where not met, compliance is generally reduced resulting in additional hazards.

While a properly placed traffic signal improves the flow and decreases accidents, an unnecessary one can be a source of danger and annoyance to all who use an intersection: Pedestrians, cyclists and drivers.