There are people who don't care at all, but most people will drive around the speed that the road encourages. That includes things like how straight the road is, what kinds of interactions, the presence of sidewalks, trees, and many other clues.

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.

I disagree, idiots are everywhere.

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.

That's true. I stopped riding the bus because the road to the stop had big speed bumps put in, and it turns out distracted drivers fly off the road when they hit them, and one near miss was enough to make me drive instead (sure it's a cognitive bias, but it's enough to make me pick the more convenient option). One fewer pedestrian means one fewer potential pedestrian death!

…plus the potential pedestrian deaths you cause by collision plus the actual impact of the extra pollution