> People in the US don't really go the speed limit even in a 25mph zone so I don't know how well that will work.
If you just change the signs, obviously it would fail miserably. You need to change the actual design of the street to match the ideal. There are lots of ways to do this, just google "road diet".
Nah, there's lots of other ways to get people to slow down. I mean, speed bumps/humps are one way, and they have their uses, but you can also add trees (yes, it's a psychological thing), narrow the road (e.g. add bike lanes or extend sidewalks), make the road less straight, add roundabouts that people can't blow through, add chicanes at some points (I've seen this in the bay area), add diverters that discourage car through-traffic, etc. Lots of options.
The most obvious difference I see between Germany and the US is that the roads are just way wider in the US. That makes drivers feel like they can go faster, so they do go faster, thus making them less safe. You don't really need to enforce tight speed limits in neighborhood streets where I live in Munich, they're so narrow that it's very obvious that you cannot go fast.
Given speed bumps are frowned upon for this very reason, narrowing roads and adding obstacles (brick and concrete islands, parking endcaps, flower beds) is preferred
If you just change the signs, obviously it would fail miserably. You need to change the actual design of the street to match the ideal. There are lots of ways to do this, just google "road diet".