I believe that a major part of this issue is that while there is a test to determine if someone is impaired from alcohol (breathalyzer), but there is no test that will show if someone is high on marijuana right at that moment. Urine and hair analysis can prove past usage, but that doesn't help the cop who pulls someone over and suspects they are under the influence, or the NTSB investigator trying to determine if a pilot was high while flying and not while he/she was on vacation in Jamaica three weeks before. Come up with a "ganja breathalyzer" and you're a lot closer to seeing legalization IMO.