I don't think it's safe. I think it should continue to be illegal. I know smoking MJ makes me a worse driver: I forget turns, miss exits, react much later to traffic control signals, and am more easily distracted by food, music, scenery, electronics etc. It also makes me very sleepy when I'm tired, accentuating the risk. I'm in my 30s, with a high tolerance; I expect the symptoms are stronger among teens who have lower tolerances, less experience controlling dosage and lower driving skills.