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Traffic Lights Warrants

This passage is extracted from http://en.wikipedia.org/wiki/Traffic_light#Traffic_signal_warrants

Traffic signals have strengths and weaknesses that must be considered when deciding whether to install them. Signaled intersections can reduce delay for side road traffic and reduce the occurrence of collisions by turning traffic and cross traffic. But they may also cause delay for traffic on the main road, and often increase rear-end collisions by up to 50%. Since right-angled and turn-against-traffic collisions are more likely to result in injuries, this is often an acceptable tradeoff. Criteria have been developed to help ensure that new signals are installed only where they will do more good than harm. In the United States these criteria are called warrants, and are found in the Manual on Uniform Traffic Control Devices (MUTCD), the Federal regulation covering the use of signs, pavement markings, traffic signals, and similar devices.

In the US MUTCD, there are nine warrants for traffic signals:
Eight-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for eight hours of an average day (usually a weekday).
Four-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for four hours of an average day.
Peak Hour Volume or Delay. This is applied only in unusual cases, such as office parks, industrial complexes, or park and ride lots that attract or discharge large numbers of vehicles in a short time, and, for a minimum of one hour of an average day. The side road traffic suffers undue delay when entering or crossing the major street.
Pedestrian Volume, if the traffic volume on a major street is so heavy that pedestrians experience excessive delay in attempting to cross it
School Crossing, if there not enough safe gaps in traffic for school children to safely cross the street
Coordinated Signal System, for places where adjacent traffic control signals do not keep traffic grouped together efficiently.
Crash Experience. The volumes in the eight- and four-hour warrants may be reduced if five or more right-angle and cross traffic turn collisions have happened at the intersection in a twelve month period.
Roadway Network. Installing a traffic control signal at some intersections might be justified to encourage concentration and organization of traffic flow on a roadway network.
Intersection Near a Grade Crossing. A traffic control signal is often justified at an intersection near a railroad crossing, in order to provide a preemption sequence to allow traffic queued up on the tracks an opportunity to clear the tracks before the train arrives.
An intersection should meet one or more of these warrants before a signal is installed. However, meeting one or more warrants does not require the installation of a traffic signal, only suggests that they may be suitable. It could be that a roundabout would work better there. There may be other unconsidered conditions that lead to traffic engineers to conclude a signal is undesirable. For example, they may decide not to install a signal at an intersection if traffic stopped by it will back up and block another, more heavily trafficked intersection. Also, if a signal meets only the peak hour warrant, the advantages during that time may not outweigh the disadvantages during the rest of the day.
Between 1979 and 1988, the city of Philadelphia removed 199 signals that did not meet any of the warrants. On average, the intersections had 24% fewer crashes after the unwarranted signals were removed.

P.S.: The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject.