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I see these LED speed signs on the side of the road, which display the approaching vehicle's speed.

What I have gathered is that they measure the time between the sending and receiving of the signals and then convert it into distance. My questions are:

  • How is it displayed for each vehicle on the lane which are also moving forward at the same time (like cars from left to right)?
  • How effective is it if the drivers just see the number on the sign change but they are not sure of which vehicle's speed the sign is currently displaying?

I have already checked out the wiki on how radar speed signs work, but they don't explain the mechanism. I would love to know how they work in detail, and how the signs address the speed issue in countries with wide roads and high traffic like India.

your speed sign

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Ritika Das
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These are almost certainly Doppler systems (addressing some comments). The transmitted beam, be it radar or lidar, is focussed and aimed to cover the approaching lane at a decent distance. If it's aimed too near the sign, the pointing angle will be significantly not parallel to the vehicle motion, leading to a $cos{\theta}$ error in the measurement. If it's aimed too far, the intent of the system is lost.

There's only one receiver element (so far as I know), so the largest amplitude received signal is the one which will be used to calculate the oncoming speed. There isn't any azimuth/elevation rejection used, and I rather doubt there's any prioritization of multiple returns. After all, if two or more vehicles are approaching at significantly different speeds, they're going to hit each other first :-) .

Carl Witthoft
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