Note that those road side systems are usually only used as a road sign and the vast majority (at least in California) use a cheaper and less accurate/repeatable

Note that those road side systems are usually only used as a road sign and the vast majority (at least in California) use a cheaper and less accurate/repeatable sensor than the ones used by police for speed infractions. This is especially pronounced on highways where the same sensor can give you several different readings at 1-10 mph less than actual speed.