The causes of traffic congestion

Traffic congestion occurs when a volume of traffic or modal split generates demand for space greater than the available road capacity. There are a number of specific circumstances which cause or aggravate congestion. Most of them reduce the capacity of a road at a given point or over a certain length, or increase the number of vehicles required for a given volume of people or goods. Beside that, traffic congestion is recurring, and is attributed to sheer weight of traffic. Most of the rest is attributed to traffic incidents, road works and weather events. Speed and flow can also affect network capacity though the relationship is complex.
Traffic research still cannot fully predict under which conditions a "traffic jam"may suddenly occur. It has been found that individual incidents such as accidents or even a single car braking heavily in a previously smooth flow may cause ripple effects which then spread out and create a sustained traffic jam .