Traffic enforcement is the key underlying issue. A firm, dynamic policy backed by the entire police force is the minimum requirement. Some of the cost will be recouped in fines. Increased traffic enforcement enables police to apprehend persons wanted on other charges, because many traffic violators are chronic offenders. Their cost is more than recovered from fines - which penalize offenders, not the general population.
Research shows that perception of apprehension is a very effective deterrent.
If people believe there is a high probability of being caught, they are far less likely to offend. This includes traffic violations as well as criminal acts. Visible, effective enforcement has been proven beyond a doubt to reduce speeding. To deter speeding, set realistic speed limits and enforce them strictly. Most people will comply.