“It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects…

Marijuana has been shown to impair performance on driving simulator tasks and on open and closed driving courses for up to approximately 3 hours. Decreased car handling performance, increased reaction times, impaired time and distance estimation, inability to maintain headway, lateral travel, subjective sleepiness, motor incoordination, and impaired sustained vigilance have all been reported.

Some drivers may actually be able to improve performance for brief periods by overcompensating for self-perceived impairment…

Low doses of THC moderately impair cognitive and psychomotor tasks associated with driving, while severe driving impairment is observed with high doses, chronic use and in combination with low doses of alcohol. The more difficult and unpredictable the task, the more likely marijuana will impair performance.”

Dec. 30, 2005 - DOT Drug and Human Performance Fact Sheet