Prior to the invention of radar, speed limits were normally enforced by clocking vehicles travelling through speed traps. Clocking a vehicle simply means timing how long it takes for the automobile to pass between two fixed landmarks along a roadway, from which the vehicle's average speed could easily be determined.

Setting up a speed trap that could provide legally satisfactory evidence was usually time consuming, however, and early speed traps were often difficult to hide. As a result, organizations such as the Amercian Automobile Association (AAA) could often keep accurate records of speed trap locations.

In the early 21st century, law enforcement utilized radar, LIDAR, planes, and automated devices. Officers may also use a method called pacing: following a car for a certain time to establish speed using the calibrated speedometer of the patrol car. Recently, Automatic Number Plate Recognition (ANPR) camera systems have been used which time a vehicle between long sections of road (approximately one mile), calculating the average speed between two points. This method eliminates the risk of heavy braking at the locations of conventional speed cameras.

Speed limit policy can affect enforcement. According to the AASHTO, "experience has ... shown that speed limits set arbitrarily below the reasonable and prudent speed perceived by the public are difficult to enforce, produce noncompliance, encourage disrespect for the law, create unnecessary antagonism toward police officers, and divert traffic to lesser routes[. Arbitrarily low limits can turn otherwise reasonable drivers into habitual speed limit violators.

Source: Wikipedia