The key difference is speed. One of the earliest speeding laws considered was the automobile cannot go faster than a horse and wagon which is about 10-15 mph.

The key difference is speed. One of the earliest speeding laws considered was the automobile cannot go faster than a horse and wagon which is about 10-15 mph. It is much easier to stop in time, or get out of the way, when at 15 mph rather than 30 mph.