Why it matters: The woonerf: 15 km/h, pedestrians on the road, restricted parking

Without a kerb you spot a playing child late, and at 30 km/h your braking distance is too long to stop in time. At 15 km/h — walking speed plus a margin — you can actually stop in time. The figure is not an arbitrary rule, but the speed at which the woonerf's shared use actually stays safe.