The first electric traffic signal was installed in Cleveland, Ohio, in 1914, and three-colour signals and coordinated 'green wave' systems followed in the 1920s in cities such as Detroit and New York. Controlling conflicting flows at intersections allowed rising volumes of motor traffic to share city streets. Traffic engineering grew into a discipline of signal timing, road capacity, parking standards and level-of-service measures. These methods increasingly governed how streets were designed and widened.
Traffic signals and the engineering profession around them made mass motoring manageable but also entrenched car-priority street design for decades.
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