Traffic problems result from either traffic overload or traffic disturbances, according to HowStuffWorks. Some of these problems are planned and thus predictable, while others occur without prior notice and can cause sudden but severe traffic congestion.
Traffic overload occurs when more cars enter a roadway than exit it, and the total number of vehicles is more than the capacity of the roadway can maintain while still allowing traffic to flow freely, explains HowStuffWorks. As additional cars enter the roadway, drivers brake in order to avoid collisions. A backward-flowing traffic wave begins, as cars further and further back on the roadway must slow their speeds. Traffic cannot flow freely until more vehicles exit than enter the roadway for a sustained period.
Planned construction can also cause traffic problems, notes HowStuffWorks. Structural incidents such as bridge collapses and large potholes can also cause traffic congestion, as can bad weather that causes drivers to slow their speeds. Traffic accidents cause problems, as cars have to shift lanes or stop moving altogether to avoid the vehicles involved in the accident. Traffic stops also cause road problems, as drivers slow or switch lanes when passing the stopped cars and law enforcement vehicles. On city streets, traffic problems can occur because of malfunctioning traffic lights, parades or demonstrations, and periods of heavy pedestrian traffic in walkways, such as during rush hours and before and after school.