I've written this before and it's unpopular among the stoner crowd but I'm against legalizing marijuana. My concern is over the draconian laws they're going to pass where you give an implied consent to a blood test for drugs the same way you now give an implied consent to the police giving you a breathalyzer test. I really don't want some moron cop with an attitude vampiring blood out of my arm because my eyes are bloodshot from swimming laps in the pool.
Police in some states (I believe Texas is one) can administer a roadside blood test already on drunk driving suspicion.
People get DUI's for driving while under the influence of drugs all the time without any set standard like BAC level for drinking and driving.
In regards to marijuana, it is metabolized extremely fast in the body. Pretty much an hour after use a blood test won't be able to determine how recently and how much marijuana was consumed. A blood test can detect the presence of marijuana in the user for nearly a week after usage, but the short term diagnostics aren't there like they are with a breathlyzer test for alcohol. There are currently zero standards for measuring marijuana intoxication and consumption levels. Current blood testing procedures can't provide that information after basically an hour of use.
I'm not sure whether the legal status of a chemical really has all that much relevance in the debate honestly. Even with illegal drugs, if there were accurate means of testing quantity ingestion and intoxication from such drugs like there is with alcohol, they would be currently employed by law enforcement. Think about that.
You're entitled to your opinion, but I think your fears of roadside vampire testing due to bloodshot eyes on the way home from the pool are rather sensational.