Marijuana
and Driving: A Review of the Scientific Evidence
It
is well established that alcohol increases accident risk.
Evidence of marijuana’s culpability in on-road driving accidents is
much less convincing.
Although
cannabis intoxication has been shown to mildly impair psychomotor
skills, this impairment does not appear to be severe or long lasting.
In driving simulator tests, this impairment is typically manifested by
subjects decreasing their driving speed and requiring greater time to
respond to emergency situations.
Nevertheless,
this impairment does not appear to play a significant role in on-road
traffic accidents. A 2002 review of seven
separate studies involving 7,934 drivers reported, “Crash
culpability studies have failed to demonstrate that drivers with
cannabinoids in the blood are significantly more likely than drug-free
drivers to be culpable in road crashes.” This result is
likely because subject under the influence of marijuana are aware of
their impairment and compensate for it accordingly, such as by slowing
down and by focusing their attention when they know a response will be
required. This reaction is just the opposite of that exhibited
by drivers under the influence of alcohol, who tend to drive in a more
risky manner proportional to their intoxication.
Today,
a large body of research exists exploring the impact of marijuana on
psychomotor skills and actual driving performance. This research
consists of driving simulator studies, on-road performance studies,
crash culpability studies, and summary reviews of the existing
evidence. To date, the result of this research is fairly
consistent: Marijuana has a measurable yet relatively mild effect on
psychomotor skills, yet it does not appear to play a significant role
in vehicle crashes, particularly when compared to alcohol. Below
is a summary of some of the existing data.
SUMMARIES
“At
the present time, the evidence to suggest an involvement of cannabis
in road crashes is scientifically unproven.
To
date …, seven studies using culpability analysis have been reported,
involving a total of 7,934 drivers. Alcohol was detected as the
only drug in 1,785 drivers, and together with cannabis in 390 drivers.
Cannabis was detected in 684 drivers, and in 294 of these it was the
only drug detected.
… The results to date of crash
culpability studies have failed to demonstrate that drivers with
cannabinoids in the blood are significantly more likely than drug-free
drivers to be culpable in road crashes. …
[In] cases in which THC was the only drug
present were analyzed, the culpability ratio was found to be not
significantly different from the no-drug group.”
REFERENCE:
G. Chesher and M. Longo. 2002. Cannabis and alcohol in motor vehicle
accidents. In: F. Grotenhermen and E. Russo (Eds.) Cannabis and
Cannabinoids: Pharmacology, Toxicology, and Therapeutic Potential.
New York: Haworth Press. Pp. 313-323.
“Cannabis leads to a more cautious style of driving, [but] it has a
negative impact on decision time and trajectory. [However,] this
in itself does not mean that drivers under the influence of cannabis
represent a traffic safety risk. … Cannabis alone,
particularly in low doses, has little effect on the skills involved in
automobile driving.”
REFERENCE: Canadian Senate Special Committee on Illegal Drugs.
2002. Cannabis: Summary Report: Our Position for a Canadian Public
Policy. Ottawa. Chapter 8: Driving Under the Influence of
Cannabis.
“This report has summarized available research on cannabis and
driving.
… Evidence of impairment from the consumption of cannabis has been
reported by studies using laboratory tests, driving simulators and
on-road observation. ... Both simulation and road trials
generally find that driving behavior shortly after consumption of
larger doses of cannabis results in (i) a more cautious driving style;
(ii) increased variability in lane position (and headway); and (iii)
longer decision times. Whereas these results
indicate a 'change' from normal conditions, they do not necessarily
reflect 'impairment' in terms of performance effectiveness since few
studies report increased accident risk.
REFERENCE: UK Department of Environment, Transport and the Regions
(Road Safety Division). 2000. Cannabis and Driving: A
Review of the Literature and Commentary. Crowthorne,
Berks: TRL Limited.
“Overall, we conclude that the weight of the evidence indicates
that:
1) There is no
evidence that consumption of cannabis alone increases the risk of
culpability for traffic crash fatalities or injuries for which
hospitalization occurs, and may reduce those risks.
2) The evidence concerning the combined effect of cannabis and
alcohol on the risk of traffic fatalities and injuries, relative to
the risk of alcohol alone, is unclear.
3) It is not possible to exclude the possibility that the use of
cannabis (with or without alcohol) leads to an increased risk of road
traffic crashes causing serious injuries and vehicle damage.”
REFERENCE: M. Bates and T. Blakely. 1999. “Role of cannabis in motor
vehicle crashes.” Epidemiologic Reviews 21: 222-232.
“In conclusion, marijuana impairs driving behavior. However,
this impairment is mitigated in that subjects under marijuana
treatment appear to perceive that they are indeed impaired.
Where they can compensate, they do, for example by not overtaking, by
slowing down and by focusing their attention when they know a response
will be required. … Effects on driving behavior are present up
to an hour after smoking but do not continue for extended periods.
With respect to comparisons between alcohol and marijuana effects,
these substances tend to differ in their effects. In contrast to the compensatory behavior
exhibited by subjects under marijuana treatment, subjects who have
received alcohol tend to drive in a more risky manner. Both
substances impair performance; however, the more cautious behavior of
subjects who have received marijuana decreases the impact of the drug
on performance, whereas the opposite holds true for alcohol.”
REFERENCE: A. Smiley. 1999. Marijuana: On-Road and Driving-Simulator
Studies. In: H. Kalant et al. (Eds) The Health Effects of Cannabis.
Toronto: Center for Addiction and Mental Health. Pp. 173-191.
“Intoxication with cannabis leads to a slight impairment of
psychomotor … function. … [However,] the impairment in
driving skills does not appear to be severe, even immediately after
taking cannabis, when subjects are tested in a driving simulator.
This may be because people intoxicated by cannabis appear to
compensate for their impairment by taking fewer risks and driving more
slowly, whereas alcohol tends to encourage people to take great risks
and drive more aggressively.”
REFERENCE: UK House of Lords Select Committee on Science and
Technology. 1998. Ninth Report.
London: United Kingdom. Chapter 4: Section 4.7.
“The evidence suggests that marijuana presents a real, but secondary
safety risk; and that alcohol is the leading drug-related accident
risk factor.”
REFERENCES: D. Gieringer. 1988. Marijuana, driving, and accident
safety. Journal of Psychoactive Drugs 20: 93-101.
Logan, M.C., Hunter,
C.E., Lokan, R.J., White, J.M., & White, M.A. (2000). The
Prevalence of Alcohol, Cannabinoids, Benzodiazepines and Stimulants
Amongst Injured Drivers and Their Role in Driver Culpability: Part II:
The Relationship Between Drug Prevalence and Drug Concentration, and
Driver Culpability. Accident Analysis and Prevention, 32,
623-32.
“Blood samples from 894 patients presenting to two Emergency
Departments for treatment of motor vehicle injur[ies] … were tested
for alcohol and other drugs.
… Based on alcohol and drug testing of the full range of patients
… alcohol is clearly the major drug associated with serious crashes
and greater injury. Patients testing
positive for illicit drugs (marijuana, opiates, and cocaine), in the
absence of alcohol, were in crashes very similar to those of patients
with neither alcohol nor drugs. When other relevant variables
were considered, these drugs were not associated with more severe
crashes or greater injury.”
REFERENCE: P. Waller et al. 1997. Crash characteristics and injuries
of victims impaired by alcohol versus illicit drugs. Accident
Analysis and Prevention 29: 817-827.
“Blood specimens were collected from a sample of 1,882 drivers from
7 states, during 14 months in the years 1990 and 1991. The
sample comprised operators of passenger cars, trucks, and motorcycles
who died within 4 hours of their crash.
… While cannabinoids were detected in 7 percent of the drivers, the
psychoactive agent THC was found in only 4 percent. … The THC-only drivers
had a responsibility rate below that of the drugfree drivers.
… While the difference was not statistically significant, there was
no indication that cannabis by itself was a cause of fatal crashes.”
REFERENCE: K. Terhune. 1992. The incidence and role of drugs in
fatally injured drivers. Washington, DC: US Department of
Transportation National Highway Traffic Safety Administration, Report
No. DOT HS 808 065.
ON-ROAD PERFORMANCE STUDIES
“Marijuana's effects on actual driving performance were assessed
in a series of three studies wherein dose-effect relationships were
measured in actual driving situations that progressively approached
reality.
… THC's effects on road-tracking after doses up to 300 µg/kg never
exceeded alcohol's at bacs of 0.08%; and, were in no way unusual
compared to many medicinal drugs. Yet, THC's effects differ
qualitatively from many other drugs, especially alcohol. Evidence from the
present and previous studies strongly suggests that alcohol encourages
risky driving whereas THC encourages greater caution, at least in
experiments. Another way THC seems to differ qualitatively from many
other drugs is that the formers users seem better able to compensate
for its adverse effects while driving under the influence.”
REFERENCE: H. Robbe. 1995. Marijuana’s effects on actual
driving performance. In: C. Kloeden and A. McLean (Eds) Alcohol, Drugs and Traffic
Safety T-95. Adelaide: Australia: HHMRC Road Research
Unit, University of Adelaide. Pp. 11-20.
“This report concerns the effects of marijuana smoking on actual
driving performance. … This program of research has shown that
marijuana, when taken alone, produces a moderate degree of driving
impairment which is related to consumed THC dose. The impairment manifests itself mainly in the ability to maintain a
lateral position on the road, but its magnitude is not exceptional in
comparison with changes produced by many medicinal drugs and alcohol.
Drivers under the influence of marijuana retain insight in their
performance and will compensate when they can, for example, by slowing
down or increasing effort. As a consequence, THC’s adverse effects
on driving performance appear relatively small.”
REFERENCE: W. Hindrik and J. Robbe and J. O’Hanlon. 1993. Marijuana and actual driving
performance. Washington, DC: US Department of
Transportation National Highway Traffic Safety Administration, Report
No. DOT HS 808 078.
DRIVING SIMULATOR STUDIES
“Overall, it is possible to conclude that cannabis has a
measurable effect on psychomotor performance, particularly tracking
ability. Its effect on higher cognitive functions, for example
divided attention tasks associated with driving, appear not to be as
critical. Drivers under the influence of cannabis seem aware
that they are impaired, and attempt to compensate for this impairment
by reducing the difficulty of the driving task, for example by driving
more slowly.
In terms of road safety, it cannot be concluded that driving under the
influence of cannabis is not a hazard, as the effects of various
aspects of driver performance are unpredictable. However, in
comparison with alcohol, the severe effects of alcohol on the higher
cognitive processes of driving are likely to make this more of a
hazard, particularly at higher blood alcohol levels.”
REFERENCE: B. Sexton et al. 2000. The influence of cannabis on
driving: A report prepared for the UK Department of the Environment,
Transport and the Regions (Road Safety Division). Crowthorne,
Berks: TRL Limited.
TABULATED SUMMARY OF SIMULATOR STUDIES OF CANNABIS AND DRIVING
Table compiled by the UK Department of
Transport (2000)
