For decades road safety was treated as a problem of driver behaviour, addressed with campaigns urging people to be careful. Countries that reduced deaths most sharply took a different view: they assumed that drivers will make mistakes and designed roads so that a mistake does not kill. Barriers separate opposing traffic, junctions are replaced by roundabouts that make high-speed collisions geometrically difficult, and speeds are held down where cars and pedestrians share space. The aim shifted from preventing errors to limiting what they cost.