"Based on the oral fluid results, more nighttime drivers (14.4%) were drug-positive then were daytime drivers (11.0%). Based on the blood test results which were administered only at nighttime, 13.8% of the drivers were drug-positive. Using the combined results of either or both oral fluid and blood tests, 16.3% of the nighttime drivers were drug-positive.
"The most commonly detected drugs were Marijuana (THC) at 8.6%, Cocaine at 3.9%, and Methamphetamine at 1.3% of nighttime drivers."
Compton, Richard and Berning, Amy, "Results of the 2007 National Roadside Survey of Alcohol and Drug Use by Drivers" (Washington, DC: Dept. of Transportation, National Highway Traffic Safety Administration, July 2009), DOT HS 811 175, p. 3.