IMO, there's not enough samples to produce this study. The rate of use of marijuana while driving is still statistically -- and radically -- low. As use becomes normalized (in other words, accepted), you'll see abuse while driving to increase as well.
But, let's say what this is really trying to do: push the Legalization activist agenda. Sorry, pot is as much a drug as meth. So is alcohol. It's disappointing to see my tax money going to support the use of either.
So you don't agree with the conclusions or methodology of a study the results of which don't fit your political agenda.