I do not think "that nearly every state in the US has a law stating that the speed limit of a road has to be set by the people driving the road". I think some states do, but it is obvious to me that RI/MA/CT do not, as the speed limits do not even come close to the average speed of drivers, let alone the 80th percentile or whatever is used for that type of law.
Actually, those 3 states do in fact have those laws.
The problem is, the municipalities that set the speed limit don't usually do a traffic study and set the speed limit accordingly.
It's actually a pretty solid way to fight a speed limit by challenging the legality of the speed limit. If the speed limit was decided without doing a traffic study, it is an illegal speed limit and therefore not valid.
There is a major problem with illegally low speed limits all over the country. But people don't know their rights and their own state laws, and so municipalities get away with generating revenue with illegal speed limits.
Speeds are measured by a radar gun or a laser gun. Both instruments are extremely accurate and provide the engineer with invaluable data when used properly. Caution should be taken that the manufacturer's instructions are followed stringently in order to insure that collected data is correct and accurate for speed zoning purposes. In most cases, speed data collection is typically conducted in a passenger car or light truck. It is important that the aforementioned vehicles are unmarked so that motorists do not perceive the recorder's presence as an enforcement activity and adjust their speeds accordingly. Also, the recording vehicle should be parked in such a way that it does not affect the speed of vehicles using the roadway, preferably being positioned off the traveled way out of plain view.
Unfortunately there's a sort of corollary to the Heisenberg uncertainty principle here...there's no way to measure speed of traffic without affecting the outcome. Also, the existing speed limit may artificially limit the speed (it sure does for me!), thereby ensuring its own continuance.
The fact that enough people will follow the speed limit no matter how unreasonably slow is why they use the 85th percentile speed, rather than the average speed. I know for a fact CT has those laws, and in those laws they even state that the speed limit does nothing to control the speed of traffic.
Having up to 50% of drivers driving the speed limit or under does not effect the 85th percentile at all.
Might be easier to think of it in the way that 15% of drivers will speed. If more than 15% of the drivers are speeding, the speed limit is too slow.