New answers tagged doppler-effect
You can use a reflector with gaps. Then the light from a car will alternate between reflecting and not reflecting at a rate dependent on their velocity towards the reflector. Please excuse my crude diagram: As the car moves right to left, gaps in the reflector will cause it to appear to flash on an off.
Due to the speed of light, the "dopler shift" would be too small, at the usual speed of the observers.
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