<P> 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 trade - off . </P> <P> 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 . </P> <P> In the US MUTCD, there are nine warrants for traffic signals: </P> <Ul> <Li> Eight - hour vehicular volume . Traffic volume must exceed prescribed minima for eight hours of an average day (usually a weekday). </Li> </Ul>

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