Finland, Norway issue speeding tickets based on a percentage of income, see e.g. http://news.bbc.co.uk/2/hi/business/3477285.stm where a €170,000 was issued in 2004. I don't know of any studies of the effects of that. But there's certainly a nice Schadenfreude effect when you invariably read about slightly rich guy in Finland getting a giant fine in just about every news publication in the world.
(If they made that the law in e.g. US I wonder if some sheriff's department would get the idea to have policemen exclusively to follow around wealthy drivers -- in the style of drug property seizures and overtuned red-light cameras)
I like that law. To avoid the kind of "social profiling" you describe you'd have to make sure that police don't have any incentive to make as much money as possible. Also, I suspect that rich people might sue and thus deter police from doing things like that.
(If they made that the law in e.g. US I wonder if some sheriff's department would get the idea to have policemen exclusively to follow around wealthy drivers -- in the style of drug property seizures and overtuned red-light cameras)