Editing Gridlock (section)

==Cause==
[[File:Gridlock (Wiesbaden, Germany).jpg|thumb|Gridlock on Kaiser-Friedrich-Ring, Wiesbaden, Germany. Vehicles on three lanes blocking each other's way.]]
[[File:7th Street gridlock afternoon.jpg|thumb|Traffic gridlock in [[Miami]], [[Florida]]. Lights are green but backups fill all the space.]]
Traditional gridlock is caused by cars entering an intersection on a green light without enough room on the other side of the intersection ''at the time of entering'' to go all the way through.  This can lead to the car being trapped in the intersection when the light turns green in the other direction.  If the same situation occurs simultaneously in multiple intersections, these cars can be trapped in the intersections indefinitely.

In many jurisdictions, drivers are therefore prohibited from entering an intersection at a green light if there is no room for them to clear the intersection. If all drivers follow this rule, gridlock is impossible.<ref>Stringer, Scott M. ''[http://www.mbpo.org/uploads/policy_reports/mbp/DONT%20BLOCK%20THE%20BOX.pdf Thinking outside the box: an analysis of Manhattan gridlock and spillback enforcement]'', Office of Manhattan Borough President, July 2006. {{Dead link|date=January 2020}}</ref>

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.  Those entering vehicles in turn back up and block the exiting vehicles.

Gridlock is sometimes cited as an example of the [[prisoner's dilemma]] (from [[game theory]]).<ref>Heath, Joseph (1999). ''[http://www.utm.utoronto.ca/~w3phl296/text/table.html Normative economics] {{Webarchive|url=https://web.archive.org/web/20071118023857/http://www.utm.utoronto.ca/~w3phl296/text/table.html |date=2007-11-18 }}'', Chapter 2, Section 3. Retrieved 2007-03-19.</ref> 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 equal cooperation.