Neighborhoods can be designed to send signals about the appropriate speed, without signs or rumble strips or speed bumps. Some people will ignore these, just as they'll ignore signs, but most drivers will do what they expect for that kind of road.
The thing is, the vast majority of people - regardless of culture - have some basic sense of self-preservation. Speeding is easy when that 30km/h road is designed like a 120km/h highway. Speeding is a lot harder when that 30km/h road has speed bumps, chicanes, bottlenecks, and is paved with bricks rather than asphalt: if you try to speed, it'll quickly feel like you need to be a professional rally driver to keep your car under control.
Deliberately making roads "unsafe" forces people to slow down, which in turn actually makes it safe.