#1507 How does a speed camera work?

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How does a speed camera work? Older types of speed cameras work out the time it takes for a car to travel between two known points. Modern speed cameras look at the wave length of radio waves or how long it takes light to return to the sensor.

The old type of speed camera are the easiest to understand because they just rely on basic math. A camera is set up on a post overlooking a road. Further down the road, a fairly long way from the camera, two lines are painted on the road. The two lines are a known distance apart, several hundred meters. At the side of the road, directed at the first and second lines are a trigger for the camera. Something like a beam of light, and the camera is triggered when a car goes through the beam of light. Even older systems used a pneumatic tube in the road that triggered the camera when a car’s weight went over it. The camera takes a picture of the car as it goes over the first line and then another picture of the car as it goes over the second line. The exact time of each photo is recorded. It is then a simple process to work out the speed the car was travelling at. To calculate the speed of an object, you need to know the length of time it took to travel a specific distance. To work it out, you divide the distance by the time. For example, if two points are 500 m apart, and a car travelled at that distance in 10 seconds, you divide 500 m by 10 s, which gives 50 m/s, which is 180 km/h. The camera has taken a photograph of the front of the car, so the license plate and the driver are visible. This is to ensure that the two photographs are of the same car and so that the car can be identified. The computer system flags a human in the police force, who checks the data and then sends out a fine to the person registered as the driver. Simple. The early speed cameras had to use regular camera film, which was obviously a huge hassle and reduced their efficiency. Modern cameras are purely digital.

More modern systems use instantaneous speed measurement, but these are not technically speed cameras, just speed detectors. They are often combined with a camera system so that they can capture an image of the offending car’s license plate and the driver, which makes them into a speed camera. These systems use LIDAR or Radar. A Radar speed detector uses a radio wave. They send out the radio wave with a known wave frequency. The radio wave hits the car driving towards the detector and bounces back. The receiver picks it up and measures the wave frequency. Because of the Doppler effect, the returning wave frequency is higher than the transmitted signal, and the height of the frequency is directly proportional to the speed of the car. The main problem with a radar system is that it sends out a wide radio wave that could pick up other cars as well. Offenders can often say that they were not the car speeding and it is difficult to prove otherwise.

LIDAR is a much more accurate system because it uses a beam of light which is very narrow. The LIDAR system technically works in the same way as older speed camera systems, but with much more accuracy. The detector sends out a pulse of light, which hits the targeted vehicle and bounces back. The speed of light is obviously known, so the computer on the detector can work out the distance to the target vehicle. The process is repeated many times, and the onboard computer knows how long it took for the car to travel a certain distance and it can work out the car’s average speed. The difference between this and a speed camera is that it happens in nanoseconds. The light travels to a car 300 meters away and back in 2000 nanoseconds, which is 0.000002 seconds. The LIDAR detector can send a huge number of these signals in a very short amount of time and work out a car’s speed.

There are other methods as well, but these are the main ones. A lot of people complain about speed cameras, but they have been shown to reduce accidents in the areas where they are installed. They are often placed in accident blackspots for this exact reason. And this is what I learned today.

Sources

https://en.wikipedia.org/wiki/Traffic_enforcement_camera

https://en.wikipedia.org/wiki/Speed_limit_enforcement

https://en.wikipedia.org/wiki/Radar_speed_gun

https://en.wikipedia.org/wiki/Lidar_traffic_enforcement