Also, people generally self-regulate down to a speed that "feels safe" for the road at hand. Sometimes this itself causes problems (the HUGE suburban streets near me constantly have speeding cars, because again, the street is huge and wide lanes make people confident to drive faster) but again, the issue there is that you need to make the road narrower, and then drivers will naturally slow down.

These mechanisms among others are much more effective at traffic calming rather than putting up more damn signs to clutter things up, and hiring more cops. But of course fixing streets costs money, and police issuing fines generates it, so it's not shocking that most municipalities go with option B.