I’ve found in the past that marijuana can improve driving by diminishing the importance of getting anywhere in a hurry. This decreases the likelihood of speeding and of taking stupid risks to maintain an unnecessary speed. It also seems like it would plausibly reduce the incidence of road rage.
However, it also lowers reaction time, impairs fine motor skills and decision making abilities, and plausibly makes people more easily distracted.
Marijuana also seems to affect people very differently depending on all kinds of contexts. To some degree, I want to say that if we could do a giant controlled experiment, marijuana would overall lessen the incidence of motor vehicle accidents. But given the sensitivity to context and individual psyches, I’m not terribly confident of that conjecture.
