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Of the many psychoactive drugs, licit and illicit, that are available and used by people who subsequently drive, marijuana may well be among the least harmful." Robbe, H. and O'Hanlon, J., Marijuana and Actual Driving Performance, Washington DC: Department of transportation (1993), p. 107.

It appears that taking a couple of tokes does impair your driving!

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"Figure 1 [absent here] demonstrates that marijuana impairs driving performance as measured by an increase in lateral position variability: all three THC doses significantly affected sdlp relative to placebo<.012, .001 & .001, for the 100, 200 & 300 g/kg conditions, respectively. The Dose by Time effect was not significant indicating that impairment after marijuana was the same in both trials. Marijuana's effects on sdlp were compared to those of alcohol obtained in a very similar study by Louwerens et al. (1987). It appeared that the effects of the various administered THC doses (100-300 g/kg) on sdlp were equivalent to those associated with bacs in the range of 0.03-0.07 g%.