First, they have to set a legal limit! Likely 5 Nanograms sounds like the magic number in most places.
Now yes marijuana is fat soluble and binds a bit different in the body than alcohol, but it still metabolizes similar. I don't believe chronic users are walking around (Who have not recently used) still having 5 nanograms in their blood systems.... I'm pretty sure they would have to had recent use to be above the 5 nanograms (Please correct me if I am wrong). Not to mention MS has likely solved this issue by looking at THC metabolites (for recent use).
Legal limit of Alcohol is .O8. With that being said, alcoholics and chronic users of alcohol can have a much higher blood level than .08 and still appear sober - and arguably be sober. Some alcoholics walk around with blood levels of 2 or 3 and are still fine - able to pass sobriety tests no problem (I worked in ER and many alcoholics had a much higher blood level than .08 and seemed totally sober).
Also, everyone metabolizes alcohol a bit differently (Fast metabolizers and slow metabolizers). But in the eyes of the law it doesn't matter what or who you are, just whether you are over the legal limit of .08.
I don't see marijuana being any different! At some point there will be national standard and a cut-off point.
Just my thoughts!
GO BLO.....