The study investigated the circumstances of each accident to assess which drivers were at fault or culpable. Drivers testing positive for marijuana were found to have no greater culpability than drug-free drivers.   In every age group, alcohol was the drug most strongly associated with crash culpability.  Cocaine users also showed higher crash culpability, especially in the age range of 21-40. 

   Significantly, marijuana-using drivers aged 41 to 60 were statistically less likely to be at fault for accidents than drug-free drivers.   Similar results have been suggested in other studies, perhaps because marijuana-using drivers tend to slow down. 

   ≥This flatly refutes the rationale for random testing of truck and bus drivers,≤ observed California NORML coordinator Dale Gieringer, ≥Urine tests for marijuana donπt reflect driving impairment.≤   Two other studies [2] have failed to find higher accident rates for drivers testing positive for marijuana in urine.

   An essential reason for these negative results is the reliance on urine tests  to detect marijuana.   Urine tests are poor indicators of impairment because they detect non-psychoactive metabolites that stay in the system for days after marijuana is smoked.  Blood tests, which measure the presence of psychoactive THC, provide a better indication of current impairment.  Usually they register positive for only a few hours after smoking, though low levels of blood THC may persist in heavy users for a day or two. Some - though not all - studies have found higher accident rates for drivers testing positive for THC in  blood.   

In another new accident study [3] - the largest yet - French researchers examined 10,748 drivers involved in fatal crashes for traces of drugs and alcohol in blood.  The study found that the presence of THC in blood was associated with a somewhat higher risk of responsibility for accidents, but significantly less so than alcohol.  The increased risk for THC was dose-dependent, ranging from 1.6 at trace levels to 3 at the highest levels (above 5 nanograms THC per milliliter of blood).  In contrast, for alcohol the risk ranged from 3 at the lowest levels (below .05% blood alcohol) to over 40 at the highest levels.   The study has proved embarrassing for drug warriors in the French government, who had prematurely rushed to pass a ≥zero tolerance≤ DUI law barring any blood traces of THC before the study was complete.  The study showed that driving with THC in blood was in fact no riskier than driving at blood alcohol levels below .05%, which is legally permitted in France.   The U.S. allows alcohol levels up to .08%.

   Numerous other studies have found that marijuana is a significantly lesser driving hazard than alcohol.   ≥Current scientific evidence shows clearly that a zero tolerance standard for THC in blood is unjustified,≤ argues California NORML coordinator Dale Gieringer. An expert panel report chaired by Dr. Franjo Grotenhermen [4] of the International Association for Cannabis Medicine concluded that levels above 3.5 to 5 nanograms per milliliter of THC in blood are generally indicative of impairment. On the other hand,  there is considerable  evidence indicating that lower levels of blood THC can be consistent with safe driving.

REFERENCES

 (1 ) Carl Soderstrom et al, ≥Crash Culpability Relative to Age and Sex for Injured Drivers Using Alcohol, Marijuana or Cocaine,≤ 49th Annual Proceedings of the Association for the Advancement of Automotive Medicine, Sept. 13-14, 2005.

(2) SR Lowenstein  and J Koziol-McLain "Drugs and traffic crash responsibility: a study of injured motorists in Colorado," J Trauma 50(2):313-30 (2001); and KLL Movig et al, "Psychoactive substance use and the risk of motor vehicle accidents" [in the Netherlands], Accident Analysis and Prevention 36: 631-6 (2004).

(3) Bernard Laumon et al, ≥Cannabis Intoxication and fatal road crashes in France: population based case-control study,≤ British Medical Journal doi:10.1136/bmj.38648.617986.1F, Dec. 2, 2005.

(4)  Franjo Grotenhermen et al., ≥Developing Science-Based Per Se Limits for Driving under the Influence of Cannabis: Findings and Recommendations by an Expert Panel,≤ (International Association for Cannabis Medicine, 2005) posted at www.canorml.org/healthfacts/DUICreport.2005.pdf.