http://www.sciencedaily.com/releases/1999/03/990325110700.htm

>While smoking marijuana does impair driving ability, it does not share alcohol's effect on judgment. Drivers on marijuana remain aware of their impairment, prompting them to slow down and drive more cautiously to compensate, she says.

>"Both substances impair performance," Smiley says. "However, the more cautious behaviour of subjects who received marijuana decreases the drug's impact on performance. Their behaviour is more appropriate to their impairment, whereas subjects who received alcohol tend to drive in a more risky manner."

I can attest to that, having seen it done multiple times while sober and intoxicated myself, and having done it myself. Are you a worse driver? For sure. You'll be crawling along at 25mph in a 45 zone. Are you more dangerous? Not at all.