There is a certain type of person who will push the limit for the limit's sake. And they will inspired others to push it. Have you sat in a line of cars with a shoulder open, when somebody decides "hey I'm just going to pass everyone on the shoulder" and once they decide that a bunch of other people jump out into the shoulder because they don't want to be "chumps" waiting while some "not chump" gets away with driving on the shoulder? Group dynamics can have a huge effect on these sorts of policies (for good and bad).