A bit of science on the risks of drug driving.
Driving under the influence of alcohol has been shown to increase the risk of involvement in road traffic collisions (RTCs) however, less is known about the effects of illicit drugs, and a clear correlation between drug concentrations and RTC risk ...
www.ncbi.nlm.nih.gov
It's long, but in short:
Methamphetamine and Ecstasy are very bad
Cocaine is very bad but might theoretically have a safe dose
Cannabis might be bad but it's a fairly small effect and hard to prove
Morphine - you're good to go