Washington state conducted through experimentation proving that frequency of use greatly reduced impairment, as well in legal states traffic incidents have shown statistically significant dips. Driving impaired is still driving impaired, and should never be done, but the risks aren't even close to alcohol or countless prescriptions out on the market. As far as any mental problems, an extremely broad, and impartial study showed without a shadow of a doubt that it only poses a risk for those with underlying/preexisting conditions. If your brain is healthy and psychosis free, weed won't give it to you.