Traffic congestion is the situation in which vehicle speeds are reduced below their maximum safe or legal speeds because of the large number of vehicles on a road or network of roads or because of obstructions to the road or network, such as accidents or construction. The safe speeds are determined by the design and condition of the roads, the condition of the vehicles, weather conditions, time of day or night, etc. Road design factors determining safe speeds include lane widths, road curvature, grades, cross-streets, and signaling, and road condition factors affecting safe speeds include the smoothness or roughness of the road surface, such as the existence of potholes.
Traffic congestion can be recurring and non-recurring. The former happens daily and is due to a lack of capacity on the road, and the latter are due to factors such as bad weather, vehicle collisions and construction.
Traffic congestion has many negative effects: (1) One is that it can consume time that people would rather spend in other places and engaged in other activities than sitting in a car or bus. (2) A second is that it can increase air pollution because internal combustion engines generally do not operate efficiently at very slow speeds and with frequent stopping and starting.
(3) A third is that it can be unhealthy for road users both because of the prolonged exposure to polluted air from nearby vehicles and because the psychological stress from congestion can raise blood pressure. (4) Congestion can also delay the response times of ambulances and other emergency vehicles. (5) It can also impede pedestrians by making it more difficult for them to cross streets. (6) In addition, it can aggravate urban sprawl by encouraging people to move to lower density areas. (7) Moreover, it creates political pressure to widen existing roads or build additional roads, even though this rarely results in a substantial long term reduction in congestion because of induced demand.
There are several ways of reducing traffic congestion and its harmful effects. They include implementing congestion tolls, making parking more difficult or costly, closing some streets or areas to road transport, and improving alternative transportation. Parking can be made more difficult or costly by raising parking fees or parking taxes, prohibiting the construction of additional parking spaces, reducing the number of parking spaces, and increasing enforcement of existing parking regulations. Congestion can also be reduced through improvements in urban design to reduce the demand for road vehicle use, especially by increasing density and walkability.