Sawz ~ back to the Donte Stallworth story:

He may have had pot in his system, but it had already been established that his blood alcohol level was over the legal limit. I doubt that even the most passionate legalize-pot advocate would argue that driving under the influence of BOTH alcohol and cannibis would be advisable, or defensible.

It should also be noted that alcohol is only detectable while the subjet is intoxicated, and exits the system immediately and completely. Marijuana will show up in a person's urine and blood for weeks after ingestion, certainly long after any effect has long since worn off. Testing positive for pot doesn't mean that you're high at the time of the test in the same way that testing for alcohol indicates drunkenness.