tradeoffs
The problem as I see it is that it is impossible to define a numerical speed limit that is valid for several km of road, under all circumstances. For a given stretch of road it may sometimes be safe at 30mph, sometimes at 40, sometimes at 20. It depends on weather, condition of vehicle, time of day, etc.
Since the fundamental objective is safety, that leaves two choices. Either an adaptive system that can judge the conditions and other variables, or a mechanical system that must target the lowest value to ensure safety in all circumstances.
Cameras fall into the second group, and in the above example would have to be set at 20. That not only causes frustration, with the potential for 'road rage' and accidents, but wastes potential road capacity. It also means that drivers, especially good drivers who are experienced at adapting to conditions, have to drive with the attention fixed on their speedo, to ensure that their driving instincts don't let them stray to, say, 22mph (only 10%). That, in itself, is dangerous, since their attention is not fully on the road and other vehicles.
Prior to the use of cameras, speed limits were enforced in general by police, who (if trained) are capable of making judgements. The speed limit could be set at, say, 30. Police could judge if someone driving at 35 in very good confitions, or on a stretch of road that was better than the average, just merited a warning, or if someone driving at 25 in bad conditions or on a stretch of road that was clearly poor, still merited prosecution for driving without due care, etc.
Overall, that solution improves traffic flow, reduces stress and likely consequences, and leaves the mechanical solutions for places where it is clearly appropriate, i.e. 'binary' infringements like drivers running red lights, or ignoring level crossing barriers.
Of course any solution involving people costs money,, and politicans have never been very good at choosing between money and safety where common sense is required.
Biting the hand that feeds IT
