The real unintended consequence is that cities ultimately don’t like to run them. They’re effective, and thus the revenue the city is expecting disappears. In they end they become costs rather than revenue sources.
Speeding also carries point penalties. Get caught a few times and your license is suspended. You can’t just pay to speed indefinitely (unless you also buy something like a get out of jail free card from the police union).
Nothing special. Slight increases in minor accidents and near misses, some minority of which will involve pedestrians or road rage violence Basically the same downsides as anything else that changes the speed via rule or enforcement rather than changing the conditions of the road (e.g. "traffic calming").
Because it only affects some drivers leading to higher variance in speed leading to more friction among traffic. Same reason everyone with a brain suggests traffic calming over changing the numbers on the signs.
A normal person sees that $500 fine as a incentive to not go that fast. But there's a certain kind of person for whom $500 is nothing compared to being able to tell the story of that time the city sold them a picture of them, complete with certificate that says they broke 100mph somewhere in the city limits, a trophy to frame and display openly in the garage next to said vehicle.
Really gotta wonder if the people that downvoted disagree that people would do such a thing, don't want to give people ideas and thus buried it, or are people who would do such a thing. Or some other thing.
