The study also finds that marijuana has the inverse effect that alcohol does on drivers.

The study also finds that marijuana has the inverse effect that alcohol does on drivers. Drivers under the influence of alcohol tend to make rash decisions and risky moves, whereas those under the influence of marijuana tend to slow down, make safer choices, and increase following distance

I think it should be required, like helmets and seat belts....
http://www.businessinsider.com/it-turns-out-that-smoking-marijuana-may-actually-make-you-a-better-driver-2011-12