Marijuana has been decriminalized in Alaska for approximately ever, and impairment exclusively from THC is extremely uncommon.  We'd probably be safer on the roads if we coupled marijuana legalization with alcohol prohibition (assuming prohibition of any substance people desire has ever worked), but obviously letting adults make their own decisions about what they put in their bodies, even when it creates potential risks to the public (or themselves) takes precedence over the government trying to bubble wrap the world to ward us against every conceivable threat or danger.