The numbers are not arbitrary. There are rules that govern the chosen numbers. Over time we learn that a 2 lane road in a commercial district with street trees

The numbers are not arbitrary. There are rules that govern the chosen numbers. Over time we learn that a 2 lane road in a commercial district with street trees is a 35 zone while a 2 lane road in a residential district is a 25 zone. I agree that at some point cars could learn the rules as well.