Because in the early days cars were not traveling at lethal speeds. Collisions are very rarely fatal to people being hit at speeds below 20mph, and somewhat eas

Because in the early days cars were not traveling at lethal speeds. Collisions are very rarely fatal to people being hit at speeds below 20mph, and somewhat easier to prevent at those speeds as well. In a real mixed use street, any driver can instinctually feel that 20 mph is recklessly fast. The problem was surrendering primacy to the cars.