[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).