Traffic congestion is a condition in transport that is characterized by slower speeds, longer trip times, and increased vehicular queueing.

Several specific circumstances can 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.

Congestion simulations and real-time observations have shown that in heavy but free flowing traffic, jams can arise spontaneously, triggered by minor events ("butterfly effects"), such as an abrupt steering maneuver by a single motorist.

Their working traffic models typically use a combination of macro-, micro- and mesoscopic features, and may add matrix entropy effects, by "platooning" groups of vehicles and by randomizing the flow patterns within individual segments of the network.

Privatization of highways and road pricing have both been proposed as measures that may reduce congestion through economic incentives and disincentives [citation needed].

Congestion can also happen due to non-recurring highway incidents, such as a crash or roadworks, which may reduce the road's capacity below normal levels.

Economist Anthony Downs argues that rush hour traffic congestion is inevitable because of the benefits of having a relatively standard work day [citation needed].

The researchers, from the University of Toronto and the London School of Economics, analyzed data from the U.S. Highway Performance and Monitoring System for 1983, 1993 and 2003, as well as information on population, employment, geography, transit, and political factors.

The implication is that building new roads and widening existing ones only results in additional traffic that continues to rise until peak congestion returns to the previous level.

[19][20] Qualitative classification of traffic is often done in the form of a six-letter A-F level of service (LOS) scale defined in the Highway Capacity Manual, a US document used (or used as a basis for national guidelines) worldwide.

Traffic congestion has a number of negative effects: Road rage is aggressive or angry behavior by a driver of an automobile or other motor vehicle.

Such behavior might include rude gestures, verbal insults, deliberately driving in an unsafe or threatening manner, or making threats.

These shooting sprees even spawned a response from the AAA Motor Club to its members on how to respond to drivers with road rage or aggressive maneuvers and gestures.

[34] City planning and urban design practices can have a huge impact on levels of future traffic congestion, though they are of limited relevance for short-term change.

[40][41] Reducing road capacity has in turn been attacked as removing free choice as well as increasing travel costs and times, placing an especially high burden on the low income residents who must commute to work.

[citation needed] Based on a survey in 2024, Brisbane is the most congested cities in Australia and 10th in the world, with drivers averagely losing 84 hours throughout the year.

[74] Despite implementation since 1997 of road space rationing by the last digit of the plate number during rush hours every weekday, traffic in this 20-million-strong city still experiences severe congestion.

In order to mitigate the aggravating congestion problem, since June 30, 2008, the road space rationing program was expanded to include and restrict trucks and light commercial vehicles.

[86] A unique Chinese phenomenon of severe traffic congestion occurs during Chunyun Period or Spring Festival travel season.

[91][92][93] The event was caused by a combination of road works and thousands of coal trucks from Inner Mongolia's coalfields that travel daily to Beijing.

"[93][94] The congestion is regarded as the worst in history by duration, and is one of the longest in length after the 175 kilometres (109 mi) long Lyon-Paris traffic jam in France on February 16, 1980.

Various causes for this include: According to a 2015 study by motor oil company Castrol, Jakarta is found to be the worst city in the world for traffic congestion.

Relying on information from TomTom navigation devices in 78 countries, the index found that drivers are stopping and starting their cars 33,240 times per year on the road.

According to a survey by Waze, traffic congestion in Metro Manila is called the "worst" in the world, after Rio de Janeiro, São Paulo, and Jakarta.

[102] It is worsened by violations of traffic laws, like illegal parking, loading and unloading, beating the red light, and wrong-way driving.

[105] The Japan International Cooperation Agency (JICA) feared that daily economic losses will reach Php 6,000,000,000 by 2030 if traffic congestion cannot be controlled.

[109] In the United Kingdom the inevitability of congestion in some urban road networks has been officially recognized since the Department for Transport set down policies based on the report Traffic in Towns in 1963: Even when everything that it is possibly to do by way of building new roads and expanding public transport has been done, there would still be, in the absence of deliberate limitation, more cars trying to move into, or within our cities than could possibly be accommodated.

The Eddington Transport Study set out the case for action to improve road and rail networks, as a "crucial enabler of sustained productivity and competitiveness".

[112] At the launch of the report Eddington told journalists and transport industry representatives introducing road pricing to encourage drivers to drive less was an "economic no-brainer".

Sustainable cities, two related challenges: high quality mobility on foot and efficient urban logistics (Part II).