Question: Is there anything more effective than a speed camera (the sort along the roadside) to work out the speed of cars?

Robert Woolfson answered on 20 Jun 2013:

A speed camera is actually a fairly simple method which just measures how long it takes a car to pass a series of marks over a set distance.

A better method is a GPS unit. The GPS in a car sat-nav is fairly accurate and will tell you how fast you are going to within a couple of kilometers per hour.

To get really accurate, you need a laser. This is actually how the newest speed guns that the police sometimes use work. Basically, you point an infra-red laser at a car and measure how long it takes to bounce off the car and come back. Because light moves so fast, you can take thousands of samples a second and get incredibly accurate speeds. Although how accurate does depend on how good the maths used to write the laser gun software is.