 thumb|upright=1.35|congestion on a city road in moscow. thumb|upright|traffic jam in los angeles, 1953 traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicular queueing. the most common example is the physical use of roads by vehicles. when traffic demand is great enough that the interaction between vehicles slows the speed of the traffic stream, this results in some congestion. as demand approaches the capacity of a road (or of the intersections along the road), extreme traffic congestion sets in. when vehicles are fully stopped for periods of time, this is colloquially known as a traffic jam or traffic snarl-up. traffic congestion can lead to drivers becoming frustrated and engaging in road rage. ==causes==  traffic congestion occurs when a volume of traffic or modal split generates demand for space greater than the available road capacity; this point is commonly termed saturation. 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. about half of u.s. traffic congestion is recurring, and is attributed to sheer weight of traffic; most of the rest is attributed to traffic incidents, road work and weather events. traffic research still cannot fully predict under which conditions a "traffic jam" (as opposed to heavy, but smoothly flowing traffic) 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 (a cascading failure) which then spread out and create a sustained traffic jam when, otherwise, normal flow might have continued for some time longer. ===mathematical theories===  some traffic engineers have attempted to apply the rules of fluid dynamics to traffic flow, likening it to the flow of a fluid in a 
Road_traffic_management Road_transport Transport_reliability