Existing centralized traffic control systems go some way toward alleviating traffic congestion and ensuring the smooth flow of vehicles through a road network. Intelligent transportation systems, however, allow traffic lights to respond to changing patterns themselves. Adaptive traffic light systems create smart intersections that control traffic in response to the patterns they observe among the vehicles using them. They can also prioritize specific forms of traffic, such as emergency vehicles or public transit. Large numbers of adaptive intersections working together produce a system in which lights change in response to traffic patterns rather than on a fixed schedule, reducing weight times and keeping traffic moving smoothly.