At the risk of sounding somewhat Nietzscheian (and pretentious) I think we all know, deep in our hearts, that ALL laws are just man-made prescriptions to keep the terrible, destructive forces of our Id from raging forwards and destroying civilization. And because laws are human in origin, they are as likely to suffer from absurdity as any other avenue of existence. Blackwind's example of the criminalization of smoking versus the acceptance of drinking is a classic example; arguments for the dangers of both are available, yet "we" choose to allow one and not the other.
We also know that folks will use the law for their own purposes (selfishness is at the core of our being whether we want to admit it or not) and those who defend a particular law usually do it because it serves their purposes; those who disagree with a law usually do so because it impedes what they consider a "freedom" or "right." I personally think that riding at 100 MPH is just fine and so long as I am doing no harm I don't think anyone should have the "right" to tell me not to do it. Yup, I might cause an accident - but then I might do that at 5 MPH turning a corner. That's the nature of accidents - they just happen.
Siggy