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(from link posted below) One study of drivers arrested for reckless driving who were not alcohol impaired did find that half of these individuals tested positive for marijuana. A second problem with these studies, however, is that a positive test for marijuana does not necessarily mean that the individual was under the influence at the time of the accident. THC stays in the blood stream much longer than other intoxicants, so a positive test may simply indicate recent use.
http://scholar.google.com/scholar?h...a+stays+in+the+blood+stream+for+approximately
There's a shitload of information to be had here.. Hours of reading.