hey i acctually drive better with a couple drinmks in me than i do when im high when im high i drive really slow and i swerve all over the road and i tend to let my eyes wander around and im always worried about the cops,.so i think i know where your grandmas coming from,alchohal and marijuana are two totaly different highs so it does effect people different maybe shes like me and just gets really paranoid to drive when shes high