Or maybe human drivers will finally start following speed limits
Designing a wide-open 4 lane boulevard then slapping a 30 mph speed limit on it is a terrible plan, yet how often we do exactly that in our cities.
Ultimately though, it does result in the desired reduction in speed. So yes, it works.
No need to make them poor visibility tho. That would be just stupid.
If you design a road like a highway: wide lanes, straight, flat, open, smooth, no traffic coming against you, you'll drive highway speeds.
If you have narrow lanes, narrow areas, trees, rough surfaces, twists, tight portions, raised areas, etc. you'll drive more carefully.
Putting up a sign with a number is just "We tried nothing and we're all out of ideas".
Most injuries and deaths by car in SF are of people outside of cars, and I suspect that the drivers who maimed/killed those people were usually in front / catching up, not following someone at high speed and had their victim jump between.
The reason people drive as fast as they feel comfortable is that speed limits are seldom and inconsistently enforced, so people are more restricted by their own notion of safety than the risk of any legal consequences.
Anyone who has spent much time there, knows that they have wide roads.
Roads that say "Take me, you gypsy stallion! Do 65 on me!" (with 25MpH speed limits).
And they have some very aggressive cops that basically hide behind hedges, waiting for free spirits.
They might not like forcing people to obey the speed limits and red-light cameras. It would be a significant hit on the bottom line.