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Traffic congestion, particularly in cities, pollutes the air, jeopardizes safety, impedes energy conservation, and results in aggravating delays. Its adverse effects on the local and national economy are pegged at $40 billion annually. Although adding more road capacity can reduce traffic congestion, states and localities can also reduce congestion through transportation control measures, such as improving public transit, encouraging employers to provide incentives for carpooling, and making better use of existing roads with effective traffic control signal systems. This report discusses (1) the benefits of traffic control signal systems; (2) the problems that state and local agencies face in implementing, operating, and maintaining effective signal systems; (3) the relationship of the current signal systems to emerging technologies like Intelligent Vehicle/Highway Systems; and (4) the role of the Federal Highway Administration in assisting state and local governments with their signal systems through reviews of plans and other measures. GAO also provides related information on the operation of traffic control signal systems, such as left and right turns on red signals and the practice of running red signals.