Determining whether a driver is under the influence of marijuana is challenging. The standardized field sobriety tests used to predict alcohol levels are ineffective for marijuana intoxication. Levels of THC and its metabolites in blood and urine are not closely correlated with marijuana intoxication. So, how can law enforcement determine when THC is impairing a person’s ability to drive? Researchers are working on brain imaging technology which may offer a more reliable method for identifying impairment from marijuana intoxication.