The analogy is specious. A vehicle is a requirement in order to have an accident whether you are drunk or not. It is not a variable that is being tested. On the other hand, smoking marijuana at some time is not a requirement for being addicted to harder drugs. It is the variable that is tested. The results, however uncomfortable they may make you, incontrovertibly suggest that marijuana can be, and is a gateway drug for many people. The disparity is not slight, it's enormous.