The most important thing would be to determine a safe level of THC in the body where coordination, comprehension and alertness are only slightly affected, much like the 0.05% BAC legal driving limit. As I understand it the current limit is effectively the lowest detectable level and the authorities see no reason to change that ATM.
If pot was legalised that would have to change but do different body types react differently to THC. I am more susceptible than most to alcohol and would never drive with even small amount of booze in my system but others claim to be unaffected at above the legal limit.
Then there is fatigue, driver aids have made some major advances there in recent years but fatigue detection isn't mainstream yet. By the time its all worked out the driverless revolution will be here and we won't have to worry about it anymore.
