Radar speed signs have highly sensitive radars that are accurate for measuring vehicle speeds. Street Dynamics radar traffic speed signs and trailers are accurate to +/- 1 mph when properly installed and configured.
There are still instances, however, where drivers notice that the speeds shown on a radar speed sign don’t match the speeds shown on their speedometers.
This can be caused by several factors. First, the speed displayed on the sign is most accurate when the vehicle and sign are within a certain narrow range of angles to each other. This explains why your speed appears to be decreasing just before you pass a radar speed sign, even when your actual speed hasn’t changed.
Secondly, the radar speed sign is designed to show the fastest speed that it can detect at any given moment. This speed may be coming from your car or a vehicle in another lane.
Third, vehicle speedometers aren’t always accurate, especially if you have changed the diameter of your tires from the original tires that were installed when the car was manufactured. And if your tires are worn down, your speedometer may read faster than your actual speed.
Law enforcement officers use their own radar or lidar systems during speed enforcement operations that function differently than the radar in speed signs, so the angles and other factors affecting sign accuracy discussed above don’t affect their readings in the same way.
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