Back when Houston was turning them off (some... 12? years ago?) There was a study showing that they increased minor accidents by people stopping suddenly at reds, rather than cruising through a stale yellow that might turn as they passed, but didn't reduce the instances of major accidents of cars plowing brazenly through a solid red into traffic.

And there was a lot of hand-in-honeypot vendors taking slices of increased traffic fines etc. It looked like it was really a wealth-distribution scheme to middlemen with government contracts at least as much as it was any kind of traffic law enforcement strategy.

I've seem more favorable studies of them in the years sense, but that was the motivation I remember for having them removed once upon a time.