Speed limits are enforced on most public roadways by authorities, with the purpose to improve driver compliance with speed limits. Methods used include roadside speed traps set up and operated by the police and automated roadside "speed camera" systems, which may incorporate the use of an automatic number plate recognition system. Traditionally, police officers used stopwatches to measure the time taken for a vehicle to cover a known distance. More recently, radar guns and automated in-vehicle systems have come into use.

A worldwide review of studies found that speed cameras led to a reduction of "11% to 44% for fatal and serious injury crashes". The UK Department for Transport estimated that cameras had led to a 22% reduction in personal injury collisions and 42% fewer people being killed or seriously injured at camera sites. The British Medical Journal recently reported that speed cameras were effective at reducing accidents and injuries in their vicinity and recommended wider deployment. An LSE study in 2017 found that "adding another 1,000 cameras to British roads could save up to 190 lives annually, reduce up to 1,130 collisions and mitigate 330 serious injuries."

History

The use of speed limits pre-dates both motorized vehicles and enforcement of the laws. Facing the invention of the automobile, many nations enacted speed limit laws, and appropriate measures to enforce them. The Locomotive Acts in the UK set speed limits for vehicles, and later codified enforcement methods. The first Locomotive Act, passed in 1861, set a speed limit of in uninhabited areas, and within towns. This act also included the value of fines for violations of the law.

Stricter regulations came in the Locomotive Act 1865, which required a man with a red flag to walk ahead of qualifying powered vehicles. The distance ahead of the pedestrian crew member was reduced to in 1878 and the vehicles were required to stop on the sight of a horse. The speed limit was effectively redundant as vehicle speeds could not exceed the speed at which a person could walk.

By 1895, some drivers of early lightweight steam-powered autocars assumed that these would be legally classed as a horseless carriage and would therefore be exempt from the need for a preceding pedestrian. A test case was brought by motoring pioneer John Henry Knight, who was subsequently convicted of using a locomotive without a licence. The Locomotives on Highways Act 1896 lifted some of the restrictions introduced by the 1865 Act, notably raising the speed limit for "light locomotives" under three tonnes to . The speed limit was lifted again by the Motor Car Act 1903 to .

A royal commission on motorcars in the UK reported in 1907 and raised concerns about the manner in which speed traps were being used to raise revenue in rural areas rather than being used to protect lives in towns. In parliamentary debates at the time it was observed that "Policemen are not stationed in the villages where there are people about who might be in danger, but are hidden in hedges or ditches by the side of the most open roads in the country" and were "manifestly absurd as a protection to the public, and they are used in many counties merely as a means of extracting money from the passing traveller in a way which reminds one of the highwaymen of the Middle Ages".

In 1905 The Automobile Association was formed to help motorists avoid police speed traps. Chief Justice, Lord Alverston brought a test court case in 1910 (Betts v Stevens) against an Automobile Association patrolman and a potentially speeding motorist—the judge ruled that where a patrolman signals to a speeding driver to slow down and thereby avoid a speed trap, that person would have committed the offence of "obstructing an officer in the course of his duty" under the Prevention of Crimes Amendment Act 1885.JA Coutts, 'Obstructing the Police' (1956) 19 MLR 411 Subsequently, the organisation developed a coded warning system which was used until the 1960s whereby a patrolman would always salute the driver of a passing car that displayed a visible AA badge unless there was a speed trap nearby, on the understanding that their officers could not be prosecuted for failing to salute.

Gatsometer BV, founded in 1958 by rally driver Maurice Gatsonides, produced the 'Gatsometer' which was described as "a revolutionary speed-measuring device". Developed initially for improving his race times, it was later marketed as a police speed enforcement tool. Gatsometer claims to have developed the first radar for use with road traffic in 1971, but this claim is undermined by evidence that radar detectors were already for sale in 1967. Gatsometer BV produced the world's first mobile speed traffic camera in 1982.

VASCAR was in use in North Carolina, New York and Indiana by February 1968.

Kevin Richardson proposed the idea of rewarding drivers travelling at or below the posted limit with a cash lottery, funded by the fines on speeding drivers. This was demonstrated in Stockholm, Sweden, in November 2010.

Methods Speed limits were originally enforced by manually timing or "clocking" vehicles travelling through "speed traps" defined between two fixed landmarks along a roadway that were a known distance apart; the vehicle's average speed was then determined by dividing the distance travelled by the time taken to travel it. Setting up a speed trap that could provide legally satisfactory evidence was usually time-consuming and error-prone, as it relied on its human operators.

The method of enforcement can be classified by: measuring the instantaneous speed at a point or the average speed between two widely spaced points some minutes or hours apart temporary, moving or permanent location human operator or automated operation all vehicles measured like a census, or only a selected sample of vehicles.

Average speed measurement

VASCAR is a device that semi-automates the timing and average speed calculation of the original manually operated "speed trap". An observer on the ground, in a vehicle or in the air simply presses a button as a vehicle passes two landmarks that are a known distance apart, typically several hundred metres.

Automatic number plate recognition (ANPR) systems that use a form of optical character recognition read the vehicle's licence or registration plate. A computer system reads vehicle registration plates at two or more fixed points along a road, usually hundreds of meters or even kilometers apart, then uses the known distance between them to calculate a vehicle's average speed. From the mean value theorem, we know that the vehicle's speed must equal its average speed at some time between the measurements. If the average speed exceeds the speed limit, then a penalty is automatically issued.

Police in some countries like France have been known to prosecute drivers for speeding, using an average speed calculated from timestamps on toll road tickets.

Speed enforcement using average speed measurement is expressly prohibited in California.CVC 40802 Speed Traps, CVC 40801 Speed Trap Prohibition

Instantaneous speed measurement

Instantaneous speed cameras measure the speed at a single point. These may either be a semi-permanent fixture or be established on a temporary basis. A variety of technologies can be used: Radar speed guns use a microwave signal that is directed at a vehicle; the Doppler effect is used to derive its speed. LIDAR speed guns utilize the time of flight of laser pulses to make a series of timestamped measurements of a vehicle's distance from the laser; the data is then used to calculate the vehicle's speed. Sensors embedded in the roadway in pairs, for example electromagnetic induction or Piezo-electric strips a set distance apart. Infra-red light sensors located perpendicular to the road, e.g. TIRTL. In the early days of vehicle speed enforcement, the police primarily used two pneumatic road tubes placed a short distance apart. This measures the time vehicles take to travel between the two tubes, and the travel speed can be derived from it. The short distance between the two tubes was called a "trap" and hence the colloquial term "speed trap" that describes any police speed monitoring location.

Pacing Officers in some jurisdictions may also use pacing, particularly where a more convenient radar speed measuring device is not available—a police vehicle's speed is matched to that of a target vehicle, and the calibrated speedometer of the patrol car used to infer the other vehicle's speed.

Cameras

In recent years many jurisdictions began using cameras to record violators. These devices detect vehicles that are exceeding the speed limit and take photos of these vehicles' license plates. A ticket is then mailed out to the registered owner.

Other Some jurisdictions, such as Australia and Ohio, allow prosecutions based on a subjective speed assessment by a police officer. In the future, there is the potential to track speed limit compliance via GPS black boxes for recidivist speeders identified in the Australian National Road Safety Strategy 2011 - 2020 section on Intelligent speed adaptation.

Effectiveness Speed cameras

Aside from the issues of legality in some countries and states and of sometime opposition the effectiveness of speed cameras is very well documented. The introduction to The Effectiveness of Speed Cameras A review of evidence by Richard Allsop includes the following in the foreword by Stephen Glaister, director of the RAC (Royal Automobile Club). "While this report fully lays out the background to the introduction of speed cameras and the need for speed limits, its job is not to justify why the national limits are what they are; a review of speed limits to see whether they are soundly based is for another day. What it has done is to show that at camera sites, speeds have been reduced, and that as a result, collisions resulting in injuries have fallen. The government has said that a decision on whether speed cameras should be funded must be taken at a local level. With the current pressure on public funds, there will be – indeed there already are – those who say that what little money there is can be better spent. This report begs to differ. The devices are already there; they demonstrate value for money, yet are not significant revenue raisers for the Treasury; they are shown to save lives; and despite the headlines, most people accept the need for them. Speed cameras should never be the only weapon in the road safety armoury, but neither should they be absent from the battle."