Gridlock is a term describing an inability to move on a transport network. The term originates from a situation possible in a grid network where intersections are blocked, prohibiting vehicles from moving 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 blocking cross traffic at an intersection. In many jurisdictions, drivers are prohibited from entering an intersection if they cannot clear it before the traffic light turns red. 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.
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.
The first appearances of the word gridlock in newspapers occurred in 1980, during a transit strike in New York City. The word is attributed to Sam Schwartz, who was the chief traffic engineer for the city's 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.