http://norml.org/index.cfm?Group_ID=5448

> Policy debates regarding marijuana-law reform invariably raise the issue of marijuana and driving. This is a valid concern. In fact, NORML's own "Principles of Responsible Cannabis Use" invoke a "no driving" clause, stating: "Although cannabis is said by most experts to be safer than alcohol and many prescription drugs with motorists, responsible cannabis consumers never operate motor vehicles in an impaired condition."

http://www.tc.gc.ca/roadsafety/tp/tp14179/menu.htm

> the evidence is very clear that a moderate or higher dose of cannabis impairs driver performance and several of the skills necessary for safe driving.

http://www.erowid.org/plants/cannabis/cannabis_driving.shtml

> Cannabis intoxication often makes smokers more aware of their impairment, causing them to slow down and become more cautious while also worsening reaction time and attention. Cannabis users often report that driving speeds are experientially 'faster' than normal: driving a given speed feels faster and more dangerous than the same speed does while sober.

I believe that my sources could not possibly be less biased than NORML and Erowid.
While I have no reservations about private use of marijuana, it's simply not a good idea to be high and driving and it's also criminal in many jurisdictions.
"It's not as bad as being drunk" is a poor argument for anything and it should be noted that driving slow does not equal driving safe, especially on a freeway.