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Revenue raising. Because....
1. Traffic fines are built into the budget for each state as a forecast. Interestingly it increases year on year (in NSW at least).
2. Speed cameras are ALWAYS near a hill and catch cars going down said hill (law says they have to be within 1.6km of blackspot - in NSW at least)
3. I have worked in the field of road safety and have seen actual accident databases. Only 5% (no, that is NOT a misprint!) of accidents in the study I did (which focused on cars running off the road and hitting trees etc.) of crashes involved a vehicle going faster than the posted speed limit.
4. The scientific study that suggests an 11th power of risk escalation (Kloeden et. al. 1997 - you know 'go 65 in a 60 zone double the risk of injury') was flawed by using improper collision modelling and insufficient baseline controls (examined this in final year thesis when I did civil engineering degree).
I have voted - however saying that mathematically the risk of speeding goes up as a 3rd-4th power relationship (using physics equations and calculus). This mathematical model assumes all things are equal, and speed is the only variable. However our road environment is a complex beast and few if any variables would remain constant from situation to situation.
Oh, BTW if you point out revenue raising to the cop who gives you the ticket you will only cause to piss him off - however, the ensuing results are entertaining....