Legal Marijuana and Drug Testing

First off let me say that I don't use marijuana nor have I ever smoked any. I am not an advocate for the legalization of marijuana but I really don't care either way. If marijuana is ever legal here in TN,I will still never use it. Anyway, with a few states now legalizing marijuana for recreational use, and other states starting to take steps in the direction for legal pot. How will this affect drug testing in the trucking industry? Obviously alcohol is a legal drug, and as long as you are consuming alcohol and not on duty, drinking and driving, etc. that is perfectly legal. Now, lets say you are home and it's legal to smoke pot where you live. If you smoke pot during your hometime, come back to work monday morning and are drug tested, what happens now? Yes, you smoked pot but you did not come to work high nor are you driving high, and smoking pot is legal where you live. Would this be the same as getting really drunk on saturday night and coming back to work monday morning? Alcohol drank 2 days before is not going to show up on a drug and alcohol screen, but the pot will.

Just wondering how this is supposed to be regulated