Traffic congestion in most cities is a daily occurrence and manifests itself by slow speeds, especially during rush hours; longer trip times; and, sometimes, complete standstills. Such delays can lead to stressed drivers, an increased risk of accidents, and higher air pollution levels. According to the Federal Highway Administration, traffic congestion has several main causes.
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According to the Federal Highway Administration, recurrent or persistent causes account for 45 per cent of traffic congestion in the United States. Recurrent causes include poor signal management, insufficient road capacity and unrestrained road demand. The results are situations where the need for road space owing to a high number of cars exceeds the actual or possible supply. Measures to mitigate congestion from this cause usually include attempts to discourage drivers to use their cars through tolls, increased vehicular and gasoline taxes, and widespread parking charges. Local authorities, meanwhile, generally are encouraged to take such measures as adjusting signal timing at bottleneck intersections.
Non-recurrent events, such as accidents or abandoned cars, cause a restriction of roadway capacity and account for 15 per cent of U.S. traffic congestion, the FHA reports. Accidents are usually caused by inattentive drivers, equipment failure or speeding, and the subsequent blocking of the road results in slow traffic and standstills.
Weather events, including icing and heavy snow, can significantly reduce highway capacities and result in traffic congestion. Drivers are also more likely to drive more slowly or to abandon their cars during serious weather events, further restricting traffic flow. During inclement weather conditions, however, drivers are more likely to leave the car at home or at work, which tends to reduce traffic. Severe weather conditions, therefore, account for only for 15 per cent of traffic congestion in the U.S., the highway agency says.
Road work is often blamed for congestion, but, the FHA says, it accounts for only 10 per cent of traffic delays in the U.S. Ironically, much road work is initiated to repair or improve roads, with the ultimate aim to reduce future congestion. On other occasions, road work initiated by utility companies is necessary to repair or install pipes and cables. Road work usually results in reduced lane capacity, slowing traffic and causing periodic standstills.
Less common but significant causes of traffic jams are natural disasters, including hurricanes, floods and earthquakes. In a disaster, roads are often lost or blocked at the same time more drivers than usual try to use them to evacuate the endangered area. Special events, including concerts, parades and political demonstrations, also can create congestion when police cordon off streets.
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- Federal Highway Administration: Using ITS Technologies & Strategies to Manage Congestion
- Puget Sound Regional Council: Congestion Management Strategies
- University of Minnesota: "Weather Impact on Traffic Conditions and Travel Time Prediction"; Lalit Sivanandan Nookala; 2006
- National Joint Utilities Group: Utilities' Street Works & the Cost of Traffic Congestion"; Phil Goodwin, 2005
- Federal Highway Administration: The Next Step in Congestion Management