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> But failing a marijuana drug test says absolutely nothing about weather you are under the influence of marijuana.

I'm not so sure of that. Failing a marijuana drug test means that there's a measurable amount of THC in your body.

Most tests measure THC metabolites, not THC, and in a heavy user these fat solvable compounds can persist for almost a year after cessation.

They are not "under the influence" for those 10 months.

You may not be so sure of that, but I am.

A "marijuana drug test" usually means testing for THC metabolites, not active THC in your blood stream. These can show up on test results a month after you ingested anything.

Imagine if tomorrow you were called in and they did a test to see if you had a single beer/glass of wine in the last month. Would you pass that test? If you would, great.. I'm willing to put down money that most people wouldn't.

There are quantitative tests for THC and its metabolites. A quantitative test can be dialed to whatever threshold you want, within the lower limit of detection for the methodology.. Demonstrate that most people are "under the influence" at a threshold value, then set policy to that value. This is just a matter of political will and influence. Many studies have been done, enough to design specific 'influence' studies (for driving, etc).

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