One can surely argue that for an increasingly large proportion of the public, the level of monetary punishment does not provide any deterrent from breaking rules. Maybe I am imagining it, but (anecdotally) I tend to see mainly expensive cars parked illegally in handicapped spots. Income proportional fines seems to me like a fair way to apply a monetary deterrent to people that now get meaningless (to them) fines.
But is it really? 10% of your income if you make $500 a month is a LOT, because it can make a difference between being able to afford rent/food or not. But 10% of your income if you make 10 million is obviously huge, but is it going to have any impact on your lifestyle? No. I love how UK does it - the fines are relatively small, but your insurance rates can go up so much you can pay a small fortune over next 3 years…
After all the point is to make people drive safely, not make money for the state.