if the goal of the fine is to deter people from committing a traffic violation, the person making $150k will not be equally deterred compared to the person making $75k. If the fine has too little impact, it no longer works as a deterrent. This is especially true for things like parking tickets, where you aren't necessarily putting yourself or others in danger like you might be for speeding (though, assuming the two people only differ in their income and all other variables -- like how willing they are to drive dangerously -- remain equal, then the point still stands).