Speed trap

Speed limit enforcement is the action taken by appropriately empowered authorities to check that road vehicles are complying with the speed limit in force on roads and highways. 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.

The perception that speed limits in a given location are being set and enforced primarily to collect revenue rather than improve traffic safety has led to controversy.

Traffic calming was built into the 1865 Locomotive Act in the UK, which set a speed limit of 2 miles per hour (3.2 km/h) in towns and 4 miles per hour (6.4 km/h) out of town, by requiring a man with a red flag to walk 60 yards (55 m) ahead of qualifying powered vehicles. The distance ahead of the pedestrian crew member was reduced to 20 yards (18 m) 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 with 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 3 tonnes to 14 miles per hour (23 km/h). The speed limit was lifted again by the Motor Car Act 1903 to 20 miles per hour (32 km/h).

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