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.
Gatso speed camera
Police officers in Bavaria checking speed with a tripod-mounted LIDAR speed gun.
Automatic traffic speed measurement in Denmark
Traffic Speed Interceptor - Vehicles with speed camera used by Bangalore Police, India
Speed limits on road traffic, as used in most countries, set the legal maximum speed at which vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum permitted speed, expressed as kilometres per hour (km/h) or miles per hour (mph) or both. Speed limits are commonly set by the legislative bodies of national or provincial governments and enforced by national or regional police and judicial authorities. Speed limits may also be variable, or in some places nonexistent, such as on most of the Autobahnen in Germany.
A road sign shows maximum and minimum speed limit for different types of vehicle on expressway in China. Speed limit signs are shown in kilometres per hour.
Historic New Hampshire speed limit sign
In 1920 a pastor was fined US$10 for "overspeeding his machine ... at a rate greater than 25 mph (40 km/h) ... in what appeared to be a reckless manner."
Example variable speed limit sign in the United States, in mph