Why not, assuming that laws against speeding exist for public safety? (That's a dubious assumption, but if it's untrue then the speed limits need to be changed until it is.)
The cost of an accident can end up being lives, not just money, so it's hard to talk about "the cost of an accident." Even if you take a hard-nosed economically rational position and assign a monetary value to each life, the typical value is something like $10 million.
The point of these fines is to deter the behavior. If you put a cap on the amount of the fine, then sufficiently wealthy people are no longer deterred. You're essentially making it so that they can break this law with impunity because they're rich.
As for giving the police the wrong incentives, if they have any incentives related to the amount of money they bring in through fines, they already have the wrong incentives. Money from traffic tickets should never end up anywhere near the police budget.