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

enter image description here

  • $\begingroup$ It would make more sense to me if these were doppler, not time of flight like a police radar gun but cannot confirm. $\endgroup$
    – DKNguyen
    Commented Mar 2, 2022 at 15:50
  • $\begingroup$ theyre about as effective as they make drivers recheck their spedometers $\endgroup$
    – Abel
    Commented Mar 3, 2022 at 1:46
  • 1
    $\begingroup$ vehicles do not know anything ... the drivers would be the ones to know $\endgroup$
    – jsotola
    Commented Mar 3, 2022 at 4:38

1 Answer 1


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 :-) .

  • $\begingroup$ the sign in my neighborhood detects multiple targets and cycles through displaying the speed of three fastest objects ... it has a range of about 250 meters $\endgroup$
    – jsotola
    Commented Mar 3, 2022 at 4:44
  • $\begingroup$ @jsotola You clearly have a high-tech government there :-) $\endgroup$ Commented Mar 3, 2022 at 15:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.