Changing it to a percent of wealth or income would encourage more rich people to hide their assets overseas.
These days it seems the wealthy will hide their assets overseas if someone so much as farts too loudly, so I don't see this as an argument against.
They have plenty of money to hire fancy lawyers and accountants to make sure their wealth remains in tact.
And they have to pay these fancy lawyers and accountants, which means that their wealth still decreases and the money is spread around. If they have to pay another, or pay an existing one more, in order to avoid these fines, that seems like a good outcome to me.
Meanwhile, the middle class would probably get hosed because they have enough to be hurt by higher fines, but not enough to defend against it or hide their assets.
Fines are supposed to hurt at least a bit; that's how they're a deterrent. There's also no guarantee that the rough amount would increase for the middle class, either, depending on the fees already in place and the equation that replaces them. A proper equation would leave fines about the same or slightly less for the middle class.
And what happens to the poor? They'd get zero fine because they have nothing and earn nothing?
The class tiers aren't "Upper, Middle, Zero". Someone with no income is extremely unlikely to have a car in the first place. You can be poor and still have some discretionary funds, though quite small; or do you think that not a single person below the poverty line has a TV? So, while the rich are paying the lawyers, Joe Poor isn't tossed in jail for failing to pay a fine that is onerous to his income, a fine he got because he didn't slow down enough on a steep downhill slope.
But wait, there's more! Fines that are more in line with your income make them a more effective deterrent for the poor, too! Imagine that you only make $2K/mo. Something happens and you get a fine for $500. You get the fine, laugh, and throw it away; there's no way you can pay this fine, you're on a shoestring budget as it is. If there was some long-term payment option maybe you could pay it, but AFAIK this isn't a common thing. In a risk vs. reward scenario, it makes more sense to keep the $500 and hope no one tracks you down (which is incredibly unlikely for that amount.)
Now imagine the fine is only $50. The risk vs. reward balance changes greatly now: It's not worth the risk of getting arrested over $50 if you can realistically pay the fine, even if that means a bit more ramen and a bit less McDonald's for the month. It not only becomes more likely to pay but also to avoid offending in the first place.
I do agree about many driving laws being too strict, or the attention paid to them imbalanced (speeding is the common stopping reason, but tailgating and failing to pass in the left lane are more likely accident causes), but even if those were improved fines would still be a problem.