The effects of marijuana on driving pose a significant public health concern. More studies on chronic marijuana use in driving are needed. The study objectives were to (1) assess differences in the Standardized Field Sobriety Test (SFST) and driving performance outcomes between chronic medical marijuana users and nonusers and (2) identify a cutoff tetrahydrocannabinol (THC) concentration above which chronic medical marijuana users demonstrate driving impairment. This prospective cross-sectional study assessed 31 chronic marijuana users and 41 nonusers. Rapid Detect Saliva Drug Screen 10-panel was administered to all participants. Participants were given a simple visual reaction time test (SVRT) and SFST consisting of the horizontal gaze nystagmus (HGN), the one leg stand (OLS), and the walk and turn (WAT) tests. The STISIM Drive M100 driving simulator assessed driving performance. Driving parameters included standard deviation of speed (SDS), deviation of mean lane position, off-road accidents, collisions, pedestrians hit, and car-following modulus, delay, and coherence. Cannabinoid blood plasma was obtained from marijuana users. Marijuana users and nonusers did not differ in age (40.06 ± 13.92 vs. 41.53 ± 15.49, P = .6782). Marijuana users were more likely to fail the SFST (P = .005) and the WAT (P = .012) and HGN (P = .001) components. Marijuana users had slower SVRT (P = .031), less SDS (P = .039), and lower modulus (P = .003). Participants with THC >2 ng/mL (P = .017) and TCH >5 ng/mL (P = .008) had lower SDS. Participants with THC >2 ng/mL (P = .021) and THC >5 ng/mL (P = .044) had decreased modulus. Chronic marijuana users had slower reaction times, deviated less in speed, and had difficulty matching a lead vehicle's speed compared to nonusers. The effects on SDS and modulus were present at cutoffs of 2 and 5 ng/mL.