cannabis + alcohol = more impairment.
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 Not the cannabis but the alcohol is the problem. Someone can legally have 1 beer an hour and still drive. This needs to change, needs to be zero tolerance on drinking and driving. It is a bad habit because it temps someone to have a few drinks and still drive.
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 Considering there is no real test for cannabis, they will drive a little and smoke a little and become much more dangerous on the road. Not that cannabis is dangerous in it self. Someone could smoke all night and drive perfectly fine, especially if they are  regular smoker.