Usually after lesser forms of control have proven ineffective. Then traffic engineers follow specific, uniform guidelines to determine whether a traffic signal is called for. They look at daily and hourly traffic volumes on the intersection approaches, including through vehicles, turning vehicles and pedestrians. They consider the number of other signals in the area and the effect a new signal will have on other streets in the vicinity. When a location meets engineering criteria, traffic signals will improve safety and operational efficiency.
Problems can result.
Signals almost always create more overall delay to drivers. In fact, minor side street traffic may experience excessive delay, particularly during off peak hours. Because of this, drivers may actually avoid the signalized intersection and switch to alternate routes or to residential streets not meant to handle through traffic. This leads to problems elsewhere in the area. People also seldom consider the cost of signals, both in public funds and out of their own pockets.