This means nothing. The big difference between alcohol and other drugs is that there is a relatively straightforward correlation between the amount of alcohol i

This means nothing. The big difference between alcohol and other drugs is that there is a relatively straightforward correlation between the amount of alcohol in the bloodstream and the current effect of alcohol on the brain. Somebody testing positive for marijuana may not be stoned at all. It could be they were stoned last week and tested positive. Furthermore, alcohol testing is much easier and is the only type of testing really done by police in the field. Police will try to get you to admit being impaired by other drugs if they stop you, but they do not have anything like the breathalyzer to test for other drugs. Current data comparing driving impairment of various drugs and alcohol is almost entirely an artifact of the difference in testing methods.