I don't care if someone does 10-15 MPH over on a highway. However doing 40 MPH in a 25 MPH zone could have fatal consequences for pedestrians, bicyclists and scooter riders. That should not be open to "local interpretation," that's one of the reasons America's roads are much more dangerous than European roads. Speed, stop light and stop sign cameras should be placed in areas of high and/or recent injury areas to ensure that people who don't have 2 tons of metal protecting them can use the streets safely.
Designing and building safe roads costs money. Of course that money should be raised primarily from taxes. But that doesn't remove the need to punish dangerous driving (red light running can still happen even on the safest street). I would personally rather pay a fine than go to jail or spend the day in the stocks and pillory.
Red lights are a whole conversation by themselves.
One of the jurisdictions local to me makes the yellow light 4 to 5 seconds long, depending on road speed, and puts a 2 or 3 second delay after one light goes red before the opposing signal goes green. They have no red light cameras for enforcement.
Another jurisdiction, adjacent to the first, has lowered their yellow light timing below 4 seconds, sometimes lowering it to the point where it takes as long to go across some intersections as the length of the time is yellow, assuming you are at the speed limit. They love red light cameras.
Until we remove the monetary incentive from enforcement, we have no business further automating it. Use points, remedial training, reduction in driving privileges, whatever, but the incentives currently benefit those who make the rules, and the result is not safer roads.
Where I live, lawmakers and police openly admit that 10-15 mph over is perfectly fine, and expected.
Unfortunately, assurances from lawmakers and police are worthless unless they're written into the law. No matter what they might say, you're still open to legal liability for going even 1 MPH over the posted limit. Worse, this attitude makes the problem more difficult to fix—once they get everyone used to going 10-15 MPH over the posted limits they can't very well update the signs with the real speeds since that would lead people to think they can go 10-15 MPH over what the new signs say.
Selective enforcement results in injustice. Any law which is routinely broken without penalty should be considered null and void. Either set the limits properly and enforce them strictly, or do away with them entirely.
Judges throw certain speeding tickets out of court for being unreasonable. (Speed limit too low, fine too high.)