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.
Did you mean an increasingly small proportion of the public? Because income inequality statistics seem to show that the rich are few in number and they are getting richer, whereas the vast majority suffer from stagnating or even declining wages. For the latter group, receiving a $500 ticket, followed by an increase in insurance premiums once that ticket goes on the driving record, is a pretty significant financial blow.