Obviously the study is not claiming that rates of THC use in general remain the same. One possible reason: the “new recruit” people who are now willing to use

Obviously the study is not claiming that rates of THC use in general remain the same. One possible reason: the “new recruit” people who are now willing to use cannabis BECAUSE it is legal are also rule-following by being willing to stay off the road after using it. Perfectly plausible to me.