think about speeding. a speeding ticket is a meaningful amount of money to most people (usually well over $100 in the us); in some states (eg Virginia) speeding turns into a criminal offense if you are going 20+ over. and yet, most people still speed sometimes and a sizeable chunk of drivers speed almost all the time. on the other hand, speed camera tickets give a $40-50 fine but they get you every time you go 12+ over the limit as you pass the camera. almost no one exceeds the limit by more than 10mph in photo enforced zones. the fine is much lower, but the consistent enforcement makes it much more effective.