I'm not sure your argument on drunk drivers really stacks up Gromit. A cannabis user driving a car is just as dangerous.
However, I do wonder on how best to sort this all out. I am warming to the idea of legalisation, not because I approve or use the stuff but because, as the Yanks found out, if you criminalize something masses use then it simply opens up a market for Mafia type criminals. In addition, if regulated, the State could ensure good quality stuff not skunk or the like and tax it to fund a mass crack (no pun intended) down on what then would be the illegal drugs. Make the penalties real hard like min 5 years for possession and 30 years for peddling it with no loop holes for right-on judges.