Certainly not a conclusion I would have expected, and there's no proven cause-effect relationship, but this does provide food for thought:
http://www.cannabisnews.org/do-medical-marijuana-laws-save-lives-on-the-road/2013/08/11/
http://www.cannabisnews.org/do-medical-marijuana-laws-save-lives-on-the-road/2013/08/11/