The reality is that "being tough on crime" and "doing something" are political points, and the use of a ticket system generates revenue. If it were law that all funds related to minor traffic infractions had to be donated to charity (without counting for taxes) you'd see a MAJOR decrease in enforcement.
If the speed limit is below that natural speed, you end up with most drivers going faster while the few follow the absurd rule.
The above is a failure of engineering. It has been well studied in academics, but few actually account for it.
I'm not arguing against raising the speed limit to the natural speed. I'm against blaming (and tailgating and generally harassing) drivers who are following the speed limit because they don't want to have to pay a fine. It seems like the blame is misplaced.
Usually because the perceived (and even the actual) danger of driving at higher speeds is minimal. A great example that I've seen is an access-controlled highway built to modern safety standards limited to 55mph, when the highway is designed to safely support upwards of 80mph.
More, increased speeds increase the carrying capacity of a highway; that is, more cars can utilize the highway in a given time period - like rush hour.
The rule of thumb is to stay ~2 seconds behind the car in front, which means the distance obviously increases linearly with speed. If you're 75 feet back at 30mph, you should be 150ft back at 60mph.
Basically to go from 60-55, 55-50, etc. takes the same amount of time, but you're traveling faster and therefore covering more distance throughout the course of the former intervals than the latter ones.