That wouldn't actually mean no effect, you need 40% of people driving to test positive for it to be no effect. It's unlikely the population driving is equivalent to the population at large - for one there's a set of responsible people who won't drive while high. For another weed use isn't randomly distributed through the population but correlated with certain subsets, which probably have a non-average rate of driving just by coincidence.

(Not that it really matters since I don't buy for a second that anywhere near 40% of people/people-driving are high at any given time. I also don't put much faith in numbers in the abstract of a a yet-to-be-published study...)