When allowing for a reasonable margin of error, speeding is also one of the fairer ways of enforcing traffic offenses. Anyone can be inattentive and touch a wheel to the lane line once. If it happens repeatedly, the driver is likely distracted or impaired. but one time can be a simple mistake that doesn't require intervention unless it leads to a crash or other conflict.

A driver exceeding the speed limit by 20%, to throw out a number, isn't likely to do so as a one-off mistake unless he misses a signed change in the speed limit. If exceeding by 20% was a one-off under an unchanged speed limit, it most likely means the driver was already exceeding the speed limit by 10-15% as a matter of course. I am much more comfortable with a driver being cited in this case.

right, but the question is whether speeding is actually that dangerous in the first place. it's not at all clear that absolute speed is a major contributor to the rate of accidents. we do know from physics that relative speed is a major factor in the severity of accidents, and this is supported by the empirical data. in particular, the probability of death when a pedestrian is struck increases sharply past ~20 mph.

I think we could have a much better discussion (and perhaps policy) if we distinguished more clearly between driving near pedestrians and driving on limited-access roads. I'm actually a big fan of speed cameras in cities and other dense areas, since they allow me to drive at what I consider a safe speed (at or below the posted limit) without worrying about cars flying past me at a high relative speed. on US highways, the situation is kind of absurd. I can exceed the posted limit by 50% without coming close to the limit of my car's mechanical grip. if I have a good line of sight and there's minimal traffic, why shouldn't I?