The International Association for Cannabis as Medicine (IACM) reported in its presentation "Developing Science-Based Per Se Limits for Driving Under the Influence of Cannabis: Finding and Recommendations by an Expert Panel," presented Sep. 2005 at the 3rd Conference on Cannabinoids in Medicine:

“Even frequent users of cannabis do not seem to have a higher accident risk than non-users, as long as they are not under the acute influence of the drug, i.e. there appear to be no extended effects of cannabis use on traffic safety beyond the period of acute impairment…

Most studies found no effect of cannabis on psychomotor function after 4 hours… a waiting period of about three hours after smoking a medium to strong social dose (15-20 mg) will be sufficient to reduce a driver’s impairment.”

Sep. 2005