Interestly enough, speed cameras and highway patrols have nothing to do with traffic safety either. Highways could easily have camera systems to monitor the exact time when cars get on and off exits to determine how fast on average they were going during their trip. These systems have been tested and work, but the fact is that actually cracking down on speeders is massively unpopular and a political non-starter.

Or you know, that there is such a thing as safe speeding, such as when overtaking or to avoid an accident, and creating a system which would require someone to slow down and become a traffic hazard in order to avoid a fine, is absurd, oddly draconian, and likely to increase accident rates?

This and the cost of deployment. Police, for all their foiles and failings are capable of judging a situation as another human would. Thus they can use their discretion in handing out tickets.

Simple fact is that speeding is not that much of a problem. Certainly people do it and it's dangerous, but most people obey speed limits within acceptable parameters and drive fairly safely. Present levels of speed enforcement are doing their job and greater levels would have a point of diminishing returns.

Such a system would obviously be a huge privacy problem, as it would provide movement profiles of every car using a highway. So, not the best alternative spending compared to phone surveillance.