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Ever Wonder About Speed Measurement?

I think of myself as a law-abiding citizen, at least when it comes to Newton’s laws of motion. Therefore, my interest in the measurement of my vehicle speed is purely scientific and not motivated by any desire to avoid a ticket. However, I'd rather not have a ticket, so that got me thinking—how do the police measure vehicle speeds?

I thought the police used radar and the Doppler effect to detect speeders. It turns out that nowadays, in British Columbia and many other places, most law enforcement agencies use laser guns (also known as LIDAR, for Light Detection And Ranging), which are much more accurate than old school radar. It's the same technology used in 3D imaging. The word laser is actually an acronym for Light Amplification by the Stimulated (not simulated as some sites say) Emission of Radiation (usually electromagnetic). 

When a police officer aims the laser gun at a speeding vehicle and pulls the trigger, an infrared laser beam hits the car. The infrared beam has a frequency of 900 to 950 nanometres, which is not visible to humans, but is similar to the infrared radiation wavelength that our bodies emit. This beam, like other forms of electromagnetic radiation, travels at 300,000 kilometres per second—that’s like travelling around the Earth 7.5 times in one second!

The beam then bounces back and is picked up by the receiver in the laser gun. The gun then times how long it takes for the signal to return and calculates how far away the vehicle is. Lasers produce a narrow range of frequencies, which reduces the chances of interference from other sources. 

This process is repeated (about 400 pulses per second) to gather data on the distance of the vehicle and the time between measurements. The slope of the line (calculated from distance/time) gives you the speed of the vehicle. 

Because the beam is not quite hitting the vehicle straight on, the calculated speed will be less than the actual speed (cosine effect), so it is more accurate if the vehicle is farther away. Fortunately, lasers can travel long distances without spreading out much, which allows them to more precisely target particular vehicles. Rain, however, can absorb and scatter the signal. To ensure the accuracy of the laser gun, the police test the calibration at the beginning and end of each shift. 

By the time you see the police with a laser gun, they will have already measured you, so slamming on your brakes won’t help. These laser guns can also detect if someone is using a laser jammer that is sending out its own beam to confuse the reception. So, the moral of the story is: drive safely.

Do you feel the need for speed? Check out this article on the future speeds of ultra-fast travel from the blogs of Science World's Future Science Leaders.