Ever noticed that the stairs leading into some underpasses around Hong Kong have an extra step? At first, I thought that the extra step was to stop water entering and flooding the subway, but if it was, then they would have done something clever instead like a smart drainage system with a duct to channel the water somewhere else, or a ramp leading up to the step. In actual fact, the step is an ingenious way to transport people to the bottom as quickly as possible.
