The term gridlock is defined as, "A state of severe road congestion arising when continuous queues of vehicles block an entire network of intersecting streets, bringing traffic in all directions to a complete standstill; a traffic jam of this kind." The term originates from a situation possible in a grid plan where intersections are blocked, preventing vehicles from either moving forwards through the intersection or backing up to an upstream intersection.
The term gridlock is also widely used to describe high traffic congestion with minimal flow (a "traffic jam"), whether or not a blocked grid system is involved. By extension, the term has been applied to situations in other fields where flow is stalled by excess demand, or in which competing interests prevent progress.
The traditional form of gridlock is caused by traffic heading in one direction across an intersection. This traffic is then stopped, by sometimes too much capacity for the roadway or an accident, blocking the intersection. The drivers in the other direction then go into the blocked intersection trying to get through. In many jurisdictions, drivers are prohibited from entering an intersection at a green light if there is no room for them to clear the intersection. If drivers follow this rule of the road, gridlock will be prevented and traffic will only be slow in the direction that is actually congested. One method of reducing gridlock is to aggressively enforce penalties for vehicles that block intersections.
Another type of gridlock can occur during traffic surges between highway on-ramps and off-ramps located within a quarter mile of each other. Traffic exiting the highway may back up and block the entering vehicles.
Gridlock is sometimes cited as an example of the prisoner's dilemma (from game theory). Mutual cooperation among drivers would give the maximum benefit (prevention of gridlock), but this may not happen because of the desire to maximize one's own benefit (shortest travel time) given the uncertainty about the other drivers' commitment to cooperation.
New York City 
In New York City, drivers who "block the box" are subject to a moving violation that comes with a US$90.00 penalty. Mayor Michael Bloomberg, noting that the ten-minute ticketing process actually contributes to overall traffic congestion, has recently asked the New York State Legislature to remove “blocking the box” from the moving violation category. This reclassification would give more traffic agents authority to write tickets and change the current ticketing procedure, which requires that the issuing officer physically stop the violating car in traffic.
Virginia Beach, Virginia 
In Virginia Beach, Virginia, roads around the oceanfront feature signs at every intersection stating "Don't Block the Box", and threatening a $200 fine.
The obvious impacts are driver frustration and trip delay. Another impact in cities is exacerbated by the presence of urban street canyons, which effectively trap air pollution and increase air pollution exposures of motorists as well as the general urban population. Noise pollution can be aggravated by excessive starting and stopping noise of gridlocked facilities.
Alleviating gridlock 
To make a traffic system less susceptible to gridlock, a traffic metering system can be introduced. These systems determine the optimal number of vehicles allowed in a traffic system, and prevent any extra vehicles from entering. This can be done with traffic control devices, such as traffic lights or warning signs, or a better public transportation system. This type of system is used in Zurich, Switzerland.
According to the New York Times the word gridlock was coined in New York City in the early 1970s. The first appearances of gridlock in newspapers occurred during the 1980 New York City transit strike. The word is attributed to Sam Schwartz, who was then the chief traffic engineer for the New York City Department of Transportation at the time of the strike. Schwartz said the word gridlock was used internally in his department during the 1970s, perhaps as early as 1971. Writing up a memo of emergency recommendations for senior officials, he recalled the words of a colleague several years earlier who had been analyzing a proposal to close Broadway to vehicular traffic. His colleague gave the plan the thumbs-down, worrying that it would simply “lock up the grid.” Schwartz was always struck by that image and titled his 1980 memo “Gridlock Prevention Plan.”  In another interview Mr. Schwartz said that he coined the term in the mid 1970s with fellow traffic engineer, Roy Cottam, who " was a little paranoid and thought he would be blamed for gridlock and so he gave me all the credit."
Historical events 
The August 2010 traffic jam in the Beijing-Zhangjiakou highway in Hebei province, China, is considered the world's worst traffic jam ever, as traffic congestion stretched more than 100 kilometres (62 mi) from August 14 to the 26, including at least 11 days of total gridlock, with some drivers spending up to 5 days to cross this stretch of highway. 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. The New York Times has called this event the "Great Chinese Gridlock of 2010."
According to Time magazine, São Paulo has the world's worst daily traffic jams. On June 10, 2009, the historical record was set with more than 182 miles (293 km) of accumulated queues out of 522 mi (835 km) being monitored.
See also 
- Box junction
- Deadlock - computer software analogy
- Journal of Transport and Land Use
- Kerner’s breakdown minimization principle
- Roadway air dispersion modeling
- Three-phase traffic theory
- Traffic congestion: Reconstruction with Kerner’s three-phase theory
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- SP registra 293 km de congestionamentos; motorista deve evitar centro expandido até as 22h
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