Thunder Bay, Ontario: US drivers involved in fatal crashes who had trace levels of cannabis in their blood or urine are less likely to have engaged in risky driving behavior than drivers who test positive for low levels of alcohol, according to case-control data published in the current issue of the Canadian Journal of Public
Investigators at Ontario's Lakeland University Public Health Program reviewed drug and alcohol data from US drivers aged 20-49 who were involved in a fatal crash from 1993 to 2003. Researchers separated motorists into 'cases' (drivers who had at least one potentially unsafe driving action recorded in relation to the crash) and 'controls' (drivers who had no such driving action recorded). Investigators sought to determine whether drivers who tested positive for cannabis but negative for alcohol were more likely to have engaged in risky driving behavior than drivers who tested negative for both pot and alcohol. Over the ten-year period, 1,647 drivers tested negative for alcohol, but tested positive for the presence of THC in their blood or urine.
Researchers concluded that alcohol-free drivers who tested positive for cannabis had a slightly elevated risk of engaging in a potentially unsafe action compared to sober drivers, even after controlling for age, sex, and prior driving record. However, investigators also reported that drivers who tested positive for low levels of alcohol (.05 BAC) possessed a significantly greater risk of engaging in such risky driving behaviors compared to sober drivers.
Investigators wrote: "The findings point to cannabis as a potential risk factor in fatal
crashes. Individuals who tested positive for cannabis but negative for alcohol had a 29 percent excess risk of having driven in a fashion that may have contributed to the crash, compared to drivers who tested negative for cannabis. ... Yet these estimates appear small compared to alcohol and some prescription medications."
Motorists who had BAC of .05 percent, a threshold well below the legal limit for drunk driving in the US, had a 101 percent excessive risk of having driven in a risky manner compared to alcohol-free drivers, authors reported. Drivers with a BAC of .10, just over the US legal limit for drunk driving, had a 200 percent excess risk.
Canadian researchers also determined that motorists who tested positive for cannabis were generally younger, male, and had a poorer driving record in the prior three years than drivers who tested negative for pot.
Responding to the study, California NORML Coordinator Dale Gieringer said: "This is the latest in a long line of studies to show that marijuana is a lesser on-road accident hazard than alcohol. Yet, ironically, federal drug policy has been more geared toward exaggerating the minor on-road risks posed by cannabis and downplaying the known risks posed by alcohol."
An analysis of French crash data published in the British Journal of Medicine in 2005 similarly reported that drivers who tested positive for cannabis in the blood were far less likely to have been involved in a fatal traffic accidents than drivers who possessed BAC levels of .05-.08.
According to previous studies of on-road crash data, past use of cannabis (as indicated by the presence of marijuana metabolites in the urine) is not associated with an elevated risk of accident. By contrast, drivers who test positive for THC in the blood above 5 ng/ml (indicating recent use of pot) are typically associated with an elevated risk of accident
compared to drivers who test negative for cannabis and/or test positive for very low levels
of THC in the blood.
THC blood levels typically fall below 5 ng/ml in recreational (non-chronic) cannabis users within 60 to 120 minutes after inhalation.
Full text of the study, "The impact of cannabis on driving," appears in the January/February 2007 issue of the Canadian Journal of Public Health.
NORML's Model Workplace Policy for Cannabis
Drivers who test positive for marijuana in urine are no more likely to cause accidents than drug-free drivers, according to a study led by Dr. Carl Soderstrom  at the University of Maryland School of Medicine (2005). The study examined 2,405 drivers hospitalized in automobile accidents from 1997 through 2001.
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 donšt reflect driving impairment." Two other studies  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  - 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  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.
(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 .