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Not sure about the radar controlled speed indicators - a lot depends on how well they are set up.Car speedometers are allowed to overindicate speed by up to 10%, but are not allowed to underindicate speed. They are usually set up to slightly overindicate. Bear in mind that tyre pressure can alter car speedometer readings, as can the condition of the road, and certainly if you change from the manufacturers recommended tyres (not in terms of brand, but in terms of size of tyres) you will expect it to increase the inaccuracy of the speedometer readings.
Nowadays in the UK with different spped limits all over the place and it can be very difficult to know what the speed limit on a particular section of road is. I feel that it is important for drivers always to be able to instantly confirm the speed limit at any moment he/she is driving the car and suggest that there should be different clolured spots in the lines on the road to indicate speed limits. The current speed limit signs can be up to half a mile apart.
This may be off the current topic a little, but I once read some interesting posts about dynamic speed limit signs. Instead of a fixed number (e.g., 35[mph] in the U.S.), the sign is a LCD whose number changes according to the traffic flow at the time. This may require cars equipped with some wireless sensors that communicate with the sign so as to allow it to compute the average car speed, and along with some other input information the sign gathers from on the road (e.g., communicating with other speed-limit signs along the road), it may be able to compute a reasonable speed limit that changes with the change in the amount of traffic congestion.To the best of my knowledge, this idea is still largely an open problem in the research community, although there exist some prototypical wireless sensor networks deployed for traffic control purposes.