Regarding driving, it's a valid concern, though I think Alcohol is worse from an impairment standpoint. Field sobriety and blood tests could be used. But even driving is a partially moot issue since legalization doesn't significantly increase the number of people driving high.
How do you feel about people driving around on prescription painkillers? All forms of impaired driving are problematic, self driving cars can't get here soon enough.
MJ is a wonder drug for some ailments and some people. It's abused by others but not nearly to the degree of alcohol. You can't overdose on it (or it requires an intentionally insane amount put more properly), it isn't chemically addictive (though some get psychologically dependent). Responsible adults should be allowed to partake without fear of arrest or imprisonment.