Genuine answers from what I've heard regarding laws at least in CA:
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There is a precedent that you should be going with the flow of traffic, which is technically safer than going the speed limit when everyone else is faster-- but mostly only on freeways; I rarely the flow going that fast on streets, so the rule doesn't apply to that one jackass going 60 in a 40 zone. And yes, you'll just get to the red faster (good cities time popular traffic lights to the speed limit so it's even stupider to speed).
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I'm pretty sure they also set the limit to what older vehicles can pull off safely (assuming you don't tailgate, anyway). They have upped the limit in some areas, however, which I found upsetting. Like, 70mph up from 55mph over a few decades, which feels fast.
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Traffic cameras are used, but there's an interesting issue on legality. If you don't acknowledge them, they can't actually say you got the ticket (they come in the mail). I forget the details, but something about seeing your accuser? LA itself more or less decriminalized them, but your mileage may vary based on local government.
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Places do, in the form of speed bumps, stop signs and curved roads. Not using those is probably more a mix of old roads and poverty-- new suburbs use windy, wide roads. Wealthy neighborhoods install speed bumps and add tons of stop signs. I don't see it in my neighborhood because "it'll hurt business" which feels like code for "you don't matter enough for safety," hence I think it's a wealth inequality issue like most things in this country.
Anyway, that's the best of my knowledge as a CA resident. Honestly, the bigger danger to me are people who tailgate, road rage, drive on sidewalks, and do stupid things to get to an offramp. That last one? Just last Wednesday I saw a guy, car totaled, who tried to hit an offramp they missed by sharp turning into it, going up a 55 degree slope, and getting t-boned by another guy going up the normal way.
Idiots, I swear...