I think a much more palatable and more effective way to enforce desired speeds would be by charging different rates for different speeds. Imagine a pricing scheme like the example below:
People respond to financial incentives, and this system seems like a much more transparent system than fines (and insurance premiums!) to cajole people into driving the optimal speed. Plus, this would allow police to focus on the truly dangerous act of driving recklessly (which could still include driving faster than conditions allow) instead of having to arbitrarily decide which driver going 10 mph over the speed limit they're going to pull over.