This is way too simplistic a solution, which is not to say that traffic fines in themselves aren't excessively simplistic.
That said, making fines fractions of a person's income does make sense relative to the way law operates normally. I mean, when you give someone, say, 10 years of jail time, you're indirectly giving rich people a much harsher monetary fine because these years are much more valuable for them in terms of the money they'll make than for poor people.
Putting this in entirely monetary terms, someone making $1m a year is in a deficit of $10m thanks to a 10-year sentence. A person making $100k a year is in a deficit of $1m thanks to the same sentence. Therefore, in monetary terms, the law already operates by being harsher to rich people in other crimes. I don't see why someone would bring up "equality before the law" to argue against this change.
“These are my principles; if you don’t like them, I have others.” – Groucho Marx