The term speed trap can refer to a point where a speed limit is strictly enforced by police. It may also refer to locations where a speed camera is posted. Alternately, the term may also refer to a speed limit that is enforced by timing how long a vehicle takes to transverse a measured distance. Cities or road sections become known as speed traps where police have a reputation for writing an unusually high number of traffic tickets, especially speeding tickets. Sometimes the posted speed limits are not easily seen; in other places, the limits are set such that many vehicles are caught. In many of these uses, the term speed trap connotes speed limit enforcement for purposes of ticket revenue or traffic deterrence instead of safety.
Speed traps have been used since the beginning of the 20th century as a means to enforce speed limits, and Britain's Automobile Association was set up specifically to notify members of such speed traps. More modern examples of speed traps include their usage in the town of Big Cabin, Oklahoma, which raised three-fourths of its revenue from traffic citations, and a significant percentage of that was related to a section of highway where the speed limit dropped from 65 mph to 45 mph. The state of Oklahoma enacted a law in 2004 that penalizes towns where the citation revenue exceeds 50% of the annual budget.
In California traffic law, evidence obtained from speed traps (as specifically defined, see "Speed trap" in California traffic law) is not admissible. Photo enforcement systems for traffic signals may measure vehicle speeds to set the beginning of the yellow signal indication phase. Some courts have ruled that this is not a speed trap.
Speed radars in France can be fixed or mobile. Mobile radars can be stand-alone (usually on a tripod) or embedded in a car.
Laser speed cameras are used by police or gendarmes bikers. They usually require a physical interception of the driver whose speed exceeded the limit, but sometimes a photograph can be taken and this can be considered a proof. Radar detectors are illegal in France. The mere existence of a radar detector in a vehicle, even if switched off or packed in luggage, attracts a fine of up to €3000, confiscation of the device, potentially also of the vehicle, and in some cases a prison sentence.
GPS-based radar alerts are legal and popular in France. The market for "radar alerters" offer is huge. "Radar alerters" are simple GPS devices that do not offer directions but only alerts when the driver arrives near some usual speed camera location. There are also many web sites that offer free "radars as Point of Interest" GPS databases for major GPS brands. There is also a device named Coyotte that comprises a GPS and a GSM telephone and with which a user can report a speed radar just by pressing a button on the device. The other users are then warned in real time when they approach such a speed radar.
Manned static speed traps are becoming more and more frequent, usually in the form of automatic cameras carried in mobile vehicles. Some speed limit enforcement is carried out by police officers in cars (both marked and unmarked) who follow offenders (often filming them) for some distance before stopping them. This allows officers to gather evidence of other traffic offences and to gauge the overall standard of the offender's driving. Drivers of vehicles with no address in the UK currently evade paying British speeding fines very easily, given that the British police do not have the power to enforce payment on the spot, but plans have been announced to change enforcement procedures.
Camera and radar detectors are currently legal to possess and install in the UK, though the actual operation of such a device is considered to be obstruction of the police in the execution of their duty. There are plans to ban them though such a ban would not be extended to GPS-based devices that merely warn drivers of fixed camera sites.
In Arizona, the use of speed traps has been limited by state laws that limit the amount a posted speed may change in a given distance.
The notion of "speed traps" is entirely different in California. Before the advent of radars, lasers and other hi-tech speed detectors, the speed of a vehicle was often determined with the help of aircraft observations by timing the moments when the vehicle passes two specific marks on a highway with known distance between them. This way was declared illegal, and for the purposes of the law the following definition was given in the California vehicle code:
The prohibition of this kind of "speed traps" followed after a series of successful defences that argued inadmissible error margin in human timing.
Subsequently, the second clause was added to the "speed trap" definition to cover inadmissible usage of "radar or other electronic devices". It considers multiple factors, such as the operation standards of devices, training of police officers, and whether the enforced speed limits were properly justified.