Drivers who test positive for the presence of THC in blood are no more likely to be involved in motor vehicle crashes than are drug-free drivers, according to a federally sponsored case-control study involving some 9,000 participants. The study, published Friday by the United States National Highway Traffic Administration (NHTSA), is the first large-scale case-control study ever conducted in the United States to assess the crash risk associated with both drugs and alcohol use by drivers.