Speed reader boards
 

I have noticed that any time I approach one of these speed reader boards the number displayed seems to jump around pretty erratically and rarely reads what my speedometer (verified accurate by gps and highway mile markers) displays. This number jumps around even more wildly when other vehicles are around travelling at different speeds or in another direction.

I presume this is why the police have mostly switched to laser, but radar is still frequently used.

To me this doesn't exactly seem to be court-worthy precision. Are these units flawed or is most radar based speed detection systems used by the police like that?

Well put it this way. When was the last time someone was caught for speeding, that wasn't actually surpassing the speed limit?

99% of the time it's beyond a reasonable doubt..

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.