Here's the answer: "In some situations, including accidents and roadside sobriety checks, blood tests may be used. Blood tests are a much better gauge of current impairment than urine tests because they detect the actual presence of THC in the system; however, they can be sensitive to other metabolites as well. Blood tests generally register positive for just a few hours after smoking, though heavy chronic smokers may be positive for a couple of days. Less sensitive are saliva tests, which register positive for about 2-4 hours after smoking." - http://norml.org/legal/drug-testing/item/drug-testing-tips#blood
I could see that happening actually. Swab’em and give’em 20 years for vehicular manslaughter!