The speed board Radar units are usually old units no longer used for real work. They often have a wide beam width too and, as a result, will display the speeds of just about anything in the beam...approaching or going away from them, sometimes at almost the same time. The newer units can be set for receding or approaching targets and can be filtered to get only the closest targets. This is why Police need a "tracking history" to help give the correct readings using things like target proximity, speed estimation and doppler shift to be able to say what vehicle gave what reading. IF you are the only vehicle near these boards and you are nice and close, they will give you a good reading. Add any of the variables I mentioned and a reading other than your own may be displayed. It's not an incorrect reading, just not the correct one for your vehicle. Laser is target specific and is designed to pick out a single vehicle from a pack. It can only be used in a stationary mode...Radar is good either stationary or moving.