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curious, if I was driving 100km/h and a car passed me doing 130, and a cop pulled me over saying I was doing 130, do the radars know the difference of the two cars? what if cop met the two travelling cars? just a hypothetical question
The radar will give a reading for the faster vehicle and the officer's eyes will tell him or her which vehicle is moving faster.
There are different modes of operation for most radar devices. Typically, they return the strongest reading. So if you had a small motorcycle overtaking a large truck, the radar would likely show the speed of the truck, even though it's slower. Some radars also have a "fast" mode, where they can be set to show the highest speed. So in the above example, in fast mode they'd ignore the stronger signal from the truck and instead show the weaker (but faster) reading from the motorcycle. Some radars also have directional modes, so that they only show the speeds of traffic heading towards or away from the radar gun.
In your example, if both vehicles were side by side and of the same size, I'd argue the radar could pick up either vehicle unless in "fast" mode. If not, it would be up to the radar operator to visually determine which of the two vehicles was travelling faster.