I find it very unlikely that you've given this a thorough cost-benefit analysis. For one thing, automobiles were invented after alcohol but before breathalyzers

I find it very unlikely that you've given this a thorough cost-benefit analysis. For one thing, automobiles were invented after alcohol but before breathalyzers, and we got along more or less just fine without having to put a bunch of people in jail for having alcohol. I see no reason why legal marijuana use would be much different. Additionally, it's self-evidently not "all about safety". That's insane. If so, why let anyone drive at all? That's another option - there's no test for if you are driving tired, but that provides similar levels of driving impairment to alcohol, plus there's already quite a bit of illegal use of marijuana. If we ban all driving, no one can kill each other with cars, problem solved, and there are no safety implications here, because driving is not critical to your safety. Police, ambulances and firefighters can still use their vehicles, because safety is the only thing that is important.