roads are a bad analogy because you don't spread crashes if you crash yourself or get crashed into. car crashes do not follow an exponential curve and aren't contagious. you always know you've been in a crash (except if you die). somebody's always at fault if there's a crash. pretty much none of these are true in case of a plague. we close roads and quarantine people because diseases are contagious and kill people or leave them disabled more or less randomly.