# How do "Your Speed" Signs work, and how do vehicles know when it's the sign's turn to display their speed?

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.

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

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.