People who drive stoned can be prone to accidents because they are very prone to get distracted by other things in the process. 3 different people at work have crashed vehicles while stoned because they were looking at various shit. One hit a mailbox, one hit a shed outside of a 7-11 cause he didn't realize he wasn't in drive, the other hit a parked car with a 4 wheeler.
On the other hand, I know one other guy, who is stoned all day, every day, that drives just fine, and I'll travel to concerts with him.