In our street there is such a cart with a scrolling message that indicates how fast you are driving. The speed is only displayed for oncoming cars. Nothing happens to cars that drive away from it. This device already indicates the speed when the car is still fairly far away. Yet I don’t see a sensor or anything further down the street. How does this device work?
Answer
There are several ways in which the speed of a vehicle can be determined. Sensors can be built into the road surface (usually piezoelectric, pressure sensitive sensors or magnetic loops). The speed of the vehicle can be measured as it passes over two consecutive sensors. This system is usually used with fixed cameras at intersections, because they can immediately detect red light ignoring.
Another technique uses radar waves emitted by the device and reflected by the vehicle. The reflected wave is slightly shifted in frequency due to the Doppler effect.
( http://www.fisica.uniud.it/~deangeli/applets/Multimedia/Waves_java/Doppler/doppler.htm )
The speed can be determined from this frequency shift.
A newer technique uses LIDAR (Laser Imaging Detection And Ranging) and measures the time difference between two short laser pulses reflected by the vehicle. The latter techniques are used with the movable speedometers and some fixed ones.
Answered by
Prof Walter Lauriks
Physics Acoustics
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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