Editing Traffic light (section)

== Justification ==
{{Globalize section|USA|2name=the United States|date=December 2010}}
Criteria have been developed to help ensure that new traffic lights are installed only where they will do more good than harm and to justify the removal of existing traffic lights where they are not warranted. They are most often placed on arterial roads at intersections with either another arterial road or a collector road, or on an expressway where an interchange is not warranted. In some situations, traffic signals can also be found on collector roads in busy settings.

The [[International Municipal Signal Association]] provides input as to standards concerning traffic signals and control devices. One example is the input the association provided for the [[Manual on Uniform Traffic Control Devices]] (MUTCD).<ref>{{Citation |title=International Municipal Signal Association |date=23 October 2021 |url=https://en.wikipedia.org/w/index.php?title=International_Municipal_Signal_Association&oldid=1051364245 |work=Wikipedia |access-date=8 July 2023 |language=en}}</ref> The MUTCD is issued by the [[Federal Highway Administration]] (FHWA) of the [[United States Department of Transportation]] (USDOT).<ref>{{Citation |title=Manual on Uniform Traffic Control Devices |date=8 July 2023 |url=https://en.wikipedia.org/w/index.php?title=Manual_on_Uniform_Traffic_Control_Devices&oldid=1164284829 |work=Wikipedia |access-date=8 July 2023 |language=en}}</ref>

In the United States, the criteria for installation of a traffic control signal are prescribed by the [[Manual on Uniform Traffic Control Devices]] (MUTCD), which defines the criteria in nine ''warrants'':<ref>[https://mutcd.fhwa.dot.gov/htm/2009/part4/part4c.htm Section 4C. Manual on Uniform Traffic Control Devices (2009 Edition)]. Mutcd.fhwa.dot.gov.</ref>
* Eight-hour vehicular volume. Traffic volume must exceed prescribed minima for eight hours of an average weekday.
* Four-hour vehicular volume. Traffic volume must exceed prescribed minima for four hours of an average weekday.
* Peak hour volume or delay. This is applied only in unusual cases, such as [[office park]]s, industrial complexes, and [[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 weekday. The side road traffic suffers undue delays when entering or crossing the major street.
* Pedestrian volume.  If the traffic volume on a major street is so heavy that pedestrians experience excessive delays in attempting to cross it.
* School crossing. If the traffic density at school crossing times exceeds one per minute which is considered to provide too few gaps in the traffic for 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.

In the US, an intersection is usually required to 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, it only suggests that they may be suitable. It could be that a [[roundabout]] would work better. There may be other unconsidered conditions that lead traffic engineers to conclude that a signal is undesirable. For example, it may be decided 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.