Editing Traffic congestion (section)

==Causes==
{{Pie chart
| caption= Causes of traffic congestion:<ref>{{Cite web |date=October 2005 |title=An Initial Assessment of Freight Bottlenecks on Highways |url=https://www.fhwa.dot.gov/policy/otps/bottlenecks/bottlenecks.pdf |url-status=live |archive-url=https://web.archive.org/web/20230601123756/https://www.fhwa.dot.gov/policy/otps/bottlenecks/bottlenecks.pdf |archive-date=June 1, 2023 |access-date=June 9, 2023 |website=Federal Highway Administration |publisher=Cambridge Systematics, Inc.}}</ref>
| label1 = [[Traffic bottleneck|Bottlenecks]]
| value1 = 40
| label2 = [[Traffic collision|Traffic incidents]]
| value2 = 25
| label3 = [[Roadworks|Work zones]]
| value3 = 10
| label4 = Bad weather
| value4 = 15
| label5 = Poor signal timing
| value5 = 5
| label6 = Special events / other
| value6 = 5
}}[[File:Traffic Jam in Egypt.jpg|thumb|Traffic jam - traffic on the Cairo-Assiut highway is blocked due to fog.]]
[[File:Traffic jam Marginal Pinheiros 6122 SAO 07 2009.jpg|thumb|Traffic congestion on Marginal Pinheiros, near downtown [[São Paulo]]. According to ''[[Time (magazine)|Time]]'' magazine, São Paulo has the world's worst traffic jams.<ref name=Times_SP/> Drivers are informed through [[variable message sign]]s that display the prevailing [[Queueing theory|queue length]].]]
[[File:Ayalon trafic congestion time lapse.webm|thumb|[[Time-lapse photography|Time lapse]] video of traffic congestion near HaShalom interchange in [[Highway 20 (Israel)|Highway 20]], Israel]]
Traffic congestion occurs when a volume of traffic generates demand for space greater than the available street capacity; this point is commonly termed [[Degree of saturation (traffic)|saturation]]. 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. About half of U.S. traffic congestion is recurring, and is attributed to sheer volume of traffic; most of the rest is attributed to traffic incidents, road work and weather events.<ref>{{cite web|title=Congestion: A National Issue|date=August 29, 2008|url=http://www.ops.fhwa.dot.gov/aboutus/opstory.htm|access-date=September 25, 2008|archive-date=July 25, 2008|archive-url=https://web.archive.org/web/20080725055811/http://www.ops.fhwa.dot.gov/aboutus/opstory.htm|url-status=dead}}</ref><ref>{{Citation|last1=Essien|first1=Aniekan|title=The Impact of Rainfall and Temperature on Peak and Off-Peak Urban Traffic|date=2018|pages=399–407|publisher=Springer International Publishing|isbn=9783319988115|last2=Petrounias|first2=Ilias|last3=Sampaio|first3=Pedro|last4=Sampaio|first4=Sandra|series=Lecture Notes in Computer Science |volume=11030 |doi=10.1007/978-3-319-98812-2_36|s2cid=52046271 |url=https://www.research.manchester.ac.uk/portal/en/publications/the-impact-of-rainfall-and-temperature-on-peak-and-offpeak-urban-traffic(dc8d3092-8b88-4ef3-97f0-f8583165308c).html|access-date=December 1, 2019|archive-date=January 26, 2020|archive-url=https://web.archive.org/web/20200126151224/https://www.research.manchester.ac.uk/portal/en/publications/the-impact-of-rainfall-and-temperature-on-peak-and-offpeak-urban-traffic(dc8d3092-8b88-4ef3-97f0-f8583165308c).html|url-status=live|url-access=subscription}}</ref> In terms of traffic operation, rainfall reduces traffic capacity and operating speeds, thereby resulting in greater congestion and road network productivity loss.

Individual incidents such as crashes 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, the normal flow might have continued for some time longer.<ref>{{cite web|url=http://www.amasci.com/amateur/traffic/traffic1.html|title=Science Hobbyist: Traffic Waves|access-date=September 29, 2003|archive-date=October 8, 2003|archive-url=https://web.archive.org/web/20031008092827/http://www.amasci.com/amateur/traffic/traffic1.html|url-status=live}}</ref>

===Separation of work and residential areas===
People often work and live in different parts of the city. Many [[workplace]]s are located in a [[central business district]] away from [[residential area]]s, resulting in workers [[commuting]]. According to a 2011 report published by the [[United States Census Bureau]], a total of 132.3 million people in the United States commuted between their work and residential areas daily.<ref>{{cite web|url=https://www.census.gov/library/publications/2011/compendia/statab/131ed/transportation.html|title=Transportation Report By USCB|access-date=May 6, 2018|archive-date=October 20, 2017|archive-url=https://web.archive.org/web/20171020182900/https://www.census.gov/library/publications/2011/compendia/statab/131ed/transportation.html|url-status=live}}</ref>

===Movement to obtain or provide goods and services===
[[File:Istanbul Otoyol 2 Richtung FSM 2.jpg|thumb|A traffic jam in [[Istanbul]], and an opportunity for two [[simit]] vendors to sell food to drivers]]
People may need to move about within the city to obtain goods and services, for instance to purchase goods or attend classes in a different part of the city. [[Brussels]], a [[Belgium|Belgian]] city with a strong service economy, has one of the worst traffic congestion in the world, wasting 74 hours in traffic in 2014.

===Economic theories===
[[File:Trafficjamdelhi.jpg|thumb|India's [[economic development in India|economic growth]] has resulted in a massive increase in the number of private vehicles on its roads overwhelming the transport infrastructure. Shown here is a traffic jam in [[Delhi]].]]

Congested roads can be seen as an example of the [[tragedy of the commons]]. Because roads in most places are free at the point of usage, there is little financial incentive for drivers not to over-use them, up to the point where traffic collapses into a jam, when demand becomes limited by [[opportunity cost]]. [[Private highway|Privatization of highways]] and [[road pricing]] have both been proposed as measures that may reduce congestion through economic incentives and disincentives {{Citation needed|date=June 2022}}. Congestion can also happen due to non-recurring highway incidents, such as a [[car accident|crash]] or [[road works|roadworks]], which may reduce the road's capacity below normal levels.
[[File:Traffic jam in Haikou, Hainan, China 01.jpg|thumb|right|upright=0.9|[[Economic history of China (1949–present)|Rapid economic growth]] in China has resulted in a massive increase in the number of private vehicles in its major cities. Shown here is a traffic jam in downtown [[Haikou]], [[Hainan Province]], China.]]
Economist [[Anthony Downs]] argues that [[rush hour]] traffic congestion is inevitable because of the benefits of having a relatively [[Business hours|standard work day]] {{Citation needed|date=April 2016}}. In a [[capitalism|capitalist]] economy, goods can be allocated either by pricing (ability to pay) or by queueing (first-come first-served); congestion is an example of the latter. Instead of the traditional solution of making the "pipe" large enough to accommodate the total demand for peak-hour vehicle travel (a supply-side solution), either by widening roadways or increasing "flow pressure" via [[automated highway system]]s, Downs advocates greater use of [[road pricing]] to reduce congestion (a demand-side solution, effectively rationing demand), in turn putting the revenues generated therefrom into [[public transportation]] projects.

A 2011 study in ''[[The American Economic Review]]'' indicates that there may be a "fundamental law of road congestion." 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. They determined that the number of vehicle-kilometers traveled (VKT) increases in direct proportion to the available lane-kilometers of roadways. 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.<ref>{{cite web|url=http://journalistsresource.org/studies/environment/transportation/fundamental-law-road-congestion-evidence-u-s-cities/|title=Fundamental Law of Road Congestion: Evidence from U.S. Cities|publisher=journalistsresource.org|date=November 17, 2014|access-date=March 6, 2012|archive-date=February 21, 2012|archive-url=https://web.archive.org/web/20120221083322/http://journalistsresource.org/studies/environment/transportation/fundamental-law-road-congestion-evidence-u-s-cities/|url-status=live}}</ref><ref>{{cite journal | last1 = Duranton | first1 = Gilles | last2 = Turner | first2 = Matthew A. | year = 2011 | title = The Fundamental Law of Road Congestion: Evidence from U.S. Cities | url = http://www.nber.org/papers/w15376.pdf | journal = American Economic Review | volume = 101 | issue = 6 | pages = 2616–52 | doi = 10.1257/aer.101.6.2616 | access-date = September 23, 2019 | archive-date = October 13, 2019 | archive-url = https://web.archive.org/web/20191013030946/https://www.nber.org/papers/w15376.pdf | url-status = live }}</ref>