The speed limit in built up areas of the UK is generally 30 miles per hour. In some areas it is 20 miles per hour and there are speed bumps to slow cars down.
On many roads however this would be impractical, as drivers slowing down to pass over the speed bumps would cause traffic flow to slow generally. Other methods exist to catch speeding drivers. The most common are shown below.
Gatso speed cameras use radar to detect cars exceeding the speed limit. The camera takes two pictures of a speeding car, 0.5 seconds apart. Distance markers on the road, 1.5 m apart, indicate the cars speed.
Truvelo speed cameras are triggered by detector cables laid in the road. Cars trigger sensors in the cables, and computers calculate the speed of the car using the time it takes a car to travel from cable to cable. Cars going faster than the speed limit are photographed.
Police radar guns can be used anywhere, and work by measuring the doppler shift between emitted and received waves.. Microwaves reflected off oncoming cars have higher frequencies than the emitted waves, and the difference in frequency can be used to find the speed of the car.
