[random_upvoter]

> 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.

[jp555]

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.

[PixelB]

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.

[niels_olson]

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).