6 mph... yeah. About that. 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. The only time I see people generally going the speed limit is in areas with speed tracking cameras.
Works great (kind of a relaxing feeling while slowly driving on the "brick"-surfaces, like a massage), but I admit that it's expensive if not building with a long-term planning in mind.
Edit: on the other hand I do absolutely hate such streets/areas that have a low speed limit (20/30 km/h) but have absolute smooth surfaces (I don't "feel" the need to drive slowly).
I know, it's stupid, but that's how my brain works...: smooth surface therefore fast, not-smooth surface therefore slow and careful :)
Bikes being the least problematic opponent group, I would be more worried by skateboard, inline-skate, etc... opposition groups.
In Switzerland we solved it (as long as it was implemented by the local community or city) by having very smooth sidewalks (but bikes still have to drive on the street as they're too fast for the sidewalk and have more than enough suspension for the normal street at normal speeds).
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".
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.