Difficult to show a causal link to cannabis given the routine tests only establish the fact that you've injested THC sometime in the last month or so. Longer for the hair strand test.
You might find this interesting:
http://www.erowid.org/plants/cannabis/cannabis_driving4.shtml
And this:
http://www.druglibrary.org/special/goode/mjsmokers7.htm
"Marijuana, it is said, impairs the ability to perform manual tasks and manipulations. For instance, it is claimed that the widespread use of the drug represents a massive danger to society because of its obvious deterioration of driving ability, thus increasing the likelihood of fatalities on the road.[10] "I ask the kiRAB," a journalist intones, in a series of articles attempting to avert marijuana use in her readers, "If you have to fly someplace, which would you rather see your pilot take, a martini or smoke a marijuana cigarette?"[11] Aside from the inaptness of the comparison (since very few drinkers can become intoxicated on one martini, while most marijuana smokers do become high on one "joint"), the striking thing about the verbal gauntlet is that the author assumes that the answer is a foregone conclusion. In fact, do we know the answer? Which is, as a result of actual tests, the safer and which is the more dangerous? Curiously, the assertion that it is far more dangerous to drive under the influence of marijuana has never been documented; it is assumed. After all, the role of alcohol in driving fatalities is only too well known; something like twenty-five thousand deaths every year from automobile accidents can more or less be directly attributed to the overindulgence of liquor. The reasoning is that if alcohol is dangerous, marijuana must, of necessity, be worse, because it is legally prohibited; moreover, the results of the two together can only be additive.
The only tests done on driving skills was completed a few months after the Boston experiments in the state of Washington. A team of researchers, including Alfred Crancer of the Washington State Department of Motor Vehicles, and James Dille, Chairman of University of Washington's Department of Pharmacology, conducted an experiment on simulated driving skills.[12] (Tests on actual driving conditions are planned.) The various driving functions were accelerator, signal, brake, speedometer, steering, and total test score; a total of 405 checks were made throughout the course of the entire experiment, so that a subject's total number of errors could range, theoretically, from zero to 405. Subjects were experienced marijuana users who were also acquainted with the liquor intoxication. They were administered the test (1) high on marijuana