The flow of traffic is always the safest speed. NHTSA and German authorites have papers on this.
It is possible to physically slow cars down with narrower roads, speed bumps, one lane chicanes, etc. It works. Very well.
Four lane split highway with a 30mph sign next to a police station? That's a speed trap and everyone knows it.
You must have dumb drag racers where you live but then again, most drag racers are dumb to begin with. Our drag racers remember where these speed bumps are and just adjust to them. You literally hear these morons slow down and rev up again in the middle of the night.
Frankly, it's the noise that's the biggest issue with me. I wouldn't care that much if they raced quietly. I'd prefer extremely heavy fines, loss of license and even prison for anyone causing excessive noise pollution in residential neighborhoods. Especially at night.