U.S. Department of Transportation,
     National Highway Traffic Safety Administration
     (DOT HS 808 078), Final Report, November 1993:

     "This program of research has shown that marijuana, when taken alone, produces a moderate degree of
     driving impairment which is related to the consumed THC dose. The impairment manifests itself mainly in the
     ability to maintain a steady 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, where they can, for example, by slowing down or increasing
     effort. As a consequence, THC's adverse effects on driving performance appear relatively small."

     ABSTRACT

     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.
     The first was conducted on a highway closed to other traffic. Subjects (24) were treated on separate
     occasions with THC 100, 200 and 300 g/kg, and placebo. They performed a 22-km road tracking test
     beginning 30 and 90 minutes after smoking. Their lateral position variability increased significantly after
     each THC dose relative to placebo in a dose-dependent manner for two hours after smoking. The second
     study was conducted on a highway in the presence of other traffic. Subjects (16) were treated with the same
     THC doses as before. They performed a 64-km road tracking test preceded and followed by 16-km car
     following tests. Results confirmed those of the previous study. Car following performance was only slightly
     impaired. The third study was conducted in high-density urban traffic. Separate groups of 16 subjects were
     treated with 100 g/kg THC and placebo; and, ethanol (mean BAC .034 g%) and placebo. Alcohol impaired
     performance relative to placebo but subjects did not perceive it. THC did not impair driving performance yet
     the subjects thought it had. These studies show that THC in single inhaled doses up to 300 g/kg has
     significant, yet not dramatic, dose-related impairing effects on driving performance.

     INTRODUCTION

     This article describes the results of a research program that was set up to determine the dose-response
     relationship between marijuana and objectively and subjectively measured aspects of real world driving; and
     to determine whether it is possible to correlate driving performance impairment with plasma concentrations
     of the drug or a metabolite. The program consisted of three driving studies in which a variety of driving tasks
     were employed, including: maintenance of a constant speed and lateral position during uninterrupted highway
     travel, following a leading car with varying speed on a highway, and city driving. A laboratory study preceded
     the driving studies for identifying the highest THC dose to be administered in the subsequent studies.

     GENERAL PROCEDURES

     Subjects in all studies were recreational users of marijuana or hashish, i.e., smoking the drug more than once
     a month, but not daily. They were all healthy, between 21 and 40 years of age, had normal weight and
     binocular acuity, and were licensed to drive an automobile. Furthermore, law enforcement authorities were
     contacted, with the volunteers' consent, to verify that they had no previous arrests or convictions for drunken
     driving or drug trafficking.
     Each subject was required to submit a urine sample immediately upon arrival at the test site. Samples were
     assayed qualitatively for the following common 'street drugs' (or metabolites): cannabinoids,
     benzodiazepines, opiates, cocaine, amphetamines and barbiturates. In addition a breath sample was analyzed
     for the presence of alcohol. Blood samples were repeatedly taken after smoking by venepuncture.
     Quantitative analysis of THC and THC-COOH in plasma was performed by gas chromatography/mass
     spectrometry (gc/ms) using deuterated cannabinoids as internal standards.

     Marijuana and placebo marijuana cigarettes were supplied by the U.S. National Institute on Drug Abuse. The
     lowest and highest THC concentrations in the marijuana cigarettes used in the studies were 1.75% and
     3.57%, respectively. Subjects smoked the administered cigarettes through a plastic holder in their customary
     fashion.
     Subjects were accompanied during every driving test by a licensed driving instructor. A redundant control
     system in the test vehicle was available for controlling the car, should emergency situations arise.
     In each study, subjects repeatedly performed certain simple laboratory tests (e.g. critical instability tracking,
     hand and posture stability), estimated their levels of intoxication and indicated their willingness to drive
     under several specified conditions of urgency. In addition, heart rate and blood pressure were measured.
     Results of these measurements are reported elsewhere (Robbe, 1994).

     LABORATORY STUDY

     Methods

     Twenty-four subjects, equally comprised of men and women, participated in this study. They were allowed to
     smoke part or all of the THC content in three cigarettes until achieving the desired psychological effect. The
     only requirement was to smoke for a period not exceeding 15 minutes. When subjects voluntarily stopped
     smoking, cigarettes were carefully extinguished and retained for subsequent gravimetric estimation of the
     amount of THC consumed.

     Results

     Six subjects consumed one cigarette, thirteen smoked two and four smoked three (data from one male
     subject were excluded from the results because no drug was found in his plasma after smoking). The average
     amount of THC consumed was 20.8 mg, after adjustment for body weight, 308 g/kg. It should be noted that
     these amounts of THC represent both the inhaled dose and the portion that was lost through pyrolysis and
     side-stream smoke during the smoking process. There were no significant differences between males and
     females, nor between frequent and infrequent users, with respect to the weight adjusted preferred dose. It
     was decided that the maximum dose for subsequent driving studies would be 300 g/kg.

     STUDY 1: DRIVING ON A RESTRICTED HIGHWAY

     Methods

     The first driving study was conducted on a highway closed to other traffic. The same twelve men and twelve
     women who participated in the laboratory study served again as the subjects. They were treated on separate
     occasions with marijuana cigarettes containing THC doses of 0 (placebo), 100, 200, and 300 g/kg. Treatments
     were administered double-blind and in a counterbalanced order. On each occasion, subjects performed a
     road-tracking test beginning 40 minutes after initiation of smoking and repeated one hour later. The test
     involved maintaining a constant speed at 90 km/h and a steady lateral position between the delineated
     boundaries of the traffic lane. Subjects drove 22 km on a primary highway and were accompanied by a
     licensed driving instructor. The primary dependent variable was the standard deviation of lateral position
     (sdlp), which has been shown to be both highly reliable and very sensitive to the influence of sedative
     medicinal drugs and alcohol. Other dependent variables were mean speed, and standard deviations of speed
     and steering wheel angle. Blood samples were taken 10 minutes before the driving tests (i.e. 30 and 90
     minutes after initiation of smoking, respectively).

     Results

     All subjects were willing and able to finish the driving tests without great difficulty. Data from one male
     subject were excluded from the results because no drug was found in his plasma after smoking.
     Figure 1 [absent here] demonstrates that marijuana impairs driving performance as measured by an increase
     in lateral position variability: all three THC doses significantly affected sdlp relative to placebo<.012, .001 &
     .001, for the 100, 200 & 300 g/kg conditions, respectively. The Dose by Time effect was not significant
     indicating that impairment after marijuana was the same in both trials. Marijuana's effects on sdlp were
     compared to those of alcohol obtained in a very similar study by Louwerens et al. (1987). It appeared that the
     effects of the various administered THC doses (100-300 g/kg) on sdlp were equivalent to those associated
     with bacs in the range of 0.03-0.07 g%. Other driving performance measures were not significantly affected
     by THC. Plasma concentrations of the drug were clearly related to the administered dose and time of blood
     sampling but unrelated to driving performance impairment. STUDY 2: DRIVING ON A NORMAL
     HIGHWAY IN TRAFFIC Methods The second driving study was conducted on a highway in the presence of
     other traffic and involved both a road-tracking and a car-following test. A new group of sixteen subjects,
     equally comprised of men and women, participated in this study. A conservative approach was chosen in
     designing the present study in order to satisfy the strictest safety requirements. That is, the study was
     conducted according to an ascending dose series design where both active drug and placebo conditions were
     administered, double-blind, at each of three THC dose levels. THC doses were the same as those used in the
     previous study, namely 100, 200, and 300 g/kg. Cigarettes appeared identical at each level of treatment
     conditions. If any subject would have reacted in an unacceptable manner to a lower dose, he/she would not
     have been permitted to receive a higher dose. The subjects began the car-following test 45 minutes after
     smoking. The test was performed on a 16 km segment of the highway and lasted about 15 minutes. After the
     conclusion of this test, subjects performed a 64-km road-tracking test on the same highway which lasted
     about 50 minutes. At the conclusion of this test, they participated again in the car-following test. Blood
     samples were taken both before the first and after the last driving test (i.e. 35 and 190 minutes after initiation
     of smoking, respectively). The road-tracking test was the same as in the previous study except for its
     duration and the presence of other traffic. The car-following test involved attempting to match velocity with,
     and maintain a constant distance from a preceding vehicle as it executed a series of deceleration/acceleration
     maneuvers. The preceding vehicle's speed would vary between 80 and 100 km/h and the subject was
     instructed to maintain a 50 m distance however the preceding vehicle's speed might vary. The duration of one
     deceleration and acceleration maneuver was approximately 50 seconds and six to eight of these maneuvers
     were executed during one test, depending upon traffic density. The subject's average reaction time to the
     movements of the preceding vehicle, mean distance and coefficient of variation of distance during maneuvers
     were taken as the dependent variables from this. Results All subjects were able to complete the series
     without suffering any untoward reaction while driving. Data from one female subject were excluded from the
     results because no drug was found in her plasma after smoking. Road-tracking performance in the standard
     test was impaired in a dose-related manner by THC and confirmed the results obtained in the previous closed
     highway study (Figure 2). The 100 g/kg dose produced a slight elevation in mean sdlp, albeit not statistically
     significant (p.13). The 200 g/kg dose produced a significant (p.023) elevation, of dubious practical relevance.
     The 300 g/kg dose produced a highly significant (p<.007) elevation which may be viewed as practically
     relevant. After marijuana smoking, subjects drove with an average speed that was only slightly lower than
     after placebo and very close to the prescribed level. In the car-following test, subjects maintained a distance
     of 45-50 m while driving in the successive placebo conditions. They lengthened mean distance by 8, 6 and 2 m
     in the corresponding THC conditions after 100, 200 and 300 g/kg, respectively. The initially large
     drug-placebo difference and its subsequent decline is a surprising result. Our explanation for this observation
     is that the subjects' caution was greatest the first time they undertook the test under the influence of THC
     and progressively less thereafter. The reaction time of the subjects to changes in the preceding vehicle's
     speed increased following THC treatment, relative to placebo. The administered THC dose was inversely
     related to the change in reaction time, as it was to distance. However, increased reaction times were partly
     due to longer distance (i.e. the longer the distance to the preceding vehicle, the more difficult it is to perceive
     changes in its speed). Statistical adjustment for this confounding variable resulted in smaller and
     non-significant increases in reaction time following marijuana treatment, the greatest impairment (0.32 s)
     being observed in the first test following the lowest THC dose (Figure 3). Distance variability followed a
     similar pattern as mean distance and reaction time; the greatest impairment was found following the lowest
     dose. As in the previous study, plasma concentrations of the drug were not related to driving impairment.
     STUDY 3: DRIVING IN URBAN TRAFFIC Methods The program proceeded into the third driving study,
     which involved tests conducted in high-density urban traffic. There were logical and safety reasons for
     restricting the THC dose to 100 g/kg. It was given to a new group of 16 regular marijuana (or hashish) users,
     along with a placebo. For comparative purposes, another group of 16 regular users of alcohol, but not
     marijuana, were treated with a modest dose of their preferred recreational drug, ethanol, and again placebo,
     before undertaking the same city driving test. Both groups were equally comprised of men and women.
     Marijuana was administered to deliver 100 g/kg THC. The driving test commenced 30 minutes after
     smoking. The alcohol dose was chosen to yield a bac approaching 0.05 g% when the driving test commenced
     45 minutes after onset of drinking. Active drug and placebo conditions were administered double-blind and in
     a counterbalanced order in each group. Blood samples were taken immediately prior to and following all
     placebo and drug driving tests (i.e. 20 and 80 minutes after initiation of smoking, or 35 and 95 minutes after
     initiation of drinking). Driving tests were conducted in daylight over a constant 17.5 km route within the city
     limits of Maastricht. Subjects drove their placebo and active-drug rides through heavy, medium and low
     density traffic on the same day of the week, and at the same time of day. Two scoring methods were
     employed in the present study. The first, a 'molecular' approach adopted from Jones (1978), involved the
     employment of a specially trained observer who applied simple and strict criteria for recording when the
     driver made or failed to make each in a series of observable responses at predetermined points along a
     chosen route. The second, a 'molar' approach, required the driving instructor acting as the safety controller
     during the tests to retrospectively rate the driver's performance using a shortened version of the Royal
     Dutch Tourist Association's Driving Proficiency Test. In total, 108 items were dichotomously scored, as
     either pass or fail. Total test performance was measured by the percentage items scored as 'pass'. Subscores
     were calculated for vehicle checks, vehicle handling, traffic maneuvers, observation and understanding of
     traffic, and turning'. This method has been applied previously to show the impairing effects of alcohol and
     diazepam (De Gier, 1979; De Gier et al., 1981). Results Data from two male subjects in the marijuana group
     were excluded from the results because neither THC nor THC-COOH was found in their plasma after
     smoking. Neither alcohol nor marijuana significantly affected driving performance measures obtained by the
     molecular approach, indicating that it may be relatively insensitive to drug-induced changes. The molar
     approach was more sensitive. Table 1 shows that a modest dose of alcohol (bac=0.034 g%) produced a
     significant impairment in city driving, relative to placebo. More specifically, alcohol impaired both vehicle
     handling and traffic maneuvers. Marijuana, administered in a dose of 100 g/kg THC, on the other hand, did
     not significantly change mean driving performance as measured by this approach. Subjects' ratings of driving
     quality and effort to accomplish the task were strikingly different from the driving instructor's ratings. Both
     groups rated their driving performance following placebo as somewhat better than 'normal'. Following the
     active drug, ratings were significantly lower (35%, p.009) in the marijuana, but not (5%, ns) in the alcohol
     group. Perceived effort to accomplish the driving test was about the same in both groups following placebo.
     Following the active drug, a significant (p.033) increase in perceived effort was reported by the marijuana, but
     not the alcohol group. Thus, there is evidence that subjects in the marijuana group were not only aware of
     their intoxicated condition, but were also attempting to compensate for it. These seem to be important
     findings. They support both the common belief that drivers become overconfident after drinking alcohol and
     investigators' suspicions that they become more cautious and self-critical after consuming low doses of THC,
     as smoked marijuana. Drug plasma concentrations were neither related to absolute driving performance
     scores nor to the changes that occurred from placebo to drug conditions. With respect to THC, these results
     confirm the findings in previous studies. They are somewhat surprising for alcohol but may be due to the
     restricted range of ethanol concentrations in the plasma of different subjects. DISCUSSION The results of
     the studies corroborate those of previous driving simulator and closed-course tests by indicating that THC in
     inhaled doses up to 300 g/kg has significant, yet not dramatic, dose-related impairing effects on driving
     performance (cf. Smiley, 1986). Standard deviation of lateral position in the road-tracking test was the most
     sensitive measure for revealing THC's adverse effects. This is because road-tracking is primarily controlled
     by an automatic information processing system which operates outside of conscious control. The process is
     relatively impervious to environmental changes but highly vulnerable to internal factors that retard the flow
     of information through the system. THC and many other drugs are among these factors. When they interfere
     with the process that restricts road-tracking error, there is little the afflicted individual can do by way of
     compensation to restore the situation. Car-following and, to a greater extent, city driving performance
     depend more on controlled information processing and are therefore more accessible for compensatory
     mechanisms that reduce the decrements or abolish them entirely. THC's effects on road-tracking after doses
     up to 300 g/kg never exceeded alcohol's at bacs of 0.08 g%; and, were in no way unusual compared to many
     medicinal drugs' (Robbe, 1994; Robbe and O'Hanlon, 1995; O'Hanlon et al., 1995). 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 former's
     users seem better able to compensate for its adverse effects while driving under the influence. Inter-subject
     correlation's between plasma concentrations of the drug and driving performance after every dose were
     essentially nil, partly due to the peculiar kinetics of THC. It enters the brain relatively rapidly, although with
     a perceptible delay relative to plasma concentrations. Once there, it remains even at a time when plasma
     concentrations approach or reach zero. As a result, performance may still be impaired at the time that plasma
     concentrations of the drug are near the detection limit. This is exactly what happened in the first driving
     study. Therefore an important practical implications of the study is that is not possible to conclude anything
     about a driver's impairment on the basis of his/her plasma concentrations of THC and THC-COOH
     determined in a single sample. Although THC's adverse effects on driving performance appeared relatively
     small in the tests employed in this program, one can still easily imagine situations where the influence of
     marijuana smoking might have a dangerous effect; i.e., emergency situations which put high demands on the
     driver's information processing capacity, prolonged monotonous driving, and after THC has been taken with
     other drugs, especially alcohol. Because these possibilities are real, the results of the present studies should
     not be considered as the final word. They should, however, serve as the point of departure for subsequent
     studies that will ultimately complete the picture of THC's effects on driving performance. REFERENCES De
     Gier JJ (1979) A subjective measurement of the influence of ethyl/alcohol in moderate doses on real driving
     performances. Blutalkohol, 16, 363-370. De Gier JJ, 't Hart BJ, Nelemans FA and Bergman H (1981)
     Psychomotor performance and real driving performance of outpatients receiving diazepam.
     Psychopharmacology, 73, 340-347. Jones MH (1978) Driver Performance Measures for the Safe
     Performance Curriculum. Traffic Safety Center, Institute of Safety and Systems Management, University of
     South California, Los Angeles, CA (DOT HS 803 461). Louwerens JW, Gloerich ABM, de Vries G,
     Brookhuis KA and O'Hanlon JF (1987). The relationship between drivers' blood alcohol concentration (bac)
     and actual driving performance during high speed travel. Pages 183-192 in PC Noordzij and R Roszbach,
     eds., Alcohol, Drugs and Traffic Safety. Proceedings of the 10th International Conference on Alcohol, Drugs
     and Traffic Safety. Excerpta Medica, Amsterdam. O'Hanlon JF, Vermeeren A, Uiterwijk MMC, van Veggel
     LMA and Swijgman HF (1995) Anxiolytics' effects on the actual driving performance of patients and healthy
     volunteers in a standardized test: an integration of three studies. Neuropsychobiology, 31:81-88. Robbe HWJ
     (1994). Influence of Marijuana on Driving. PhD thesis,Institute for Human Psychopharmacology, University
     of Limburg, Maastricht. Robbe HWJ and O'Hanlon JF (1995) Acute and subchronic effects of paroxetine and
     amitriptyline on actual driving, psychomotor performance and subjective assessments in healthy volunteers.
     European Neuropsychopharmacology, 5:35-42 Smiley AM (1986). Marijuana: On-road and driving simulator
     studies. Alcohol, Drugs and Driving: Abstracts and Reviews 2: 121-134. Exposing Marijuana Myths Claim
     #12 : Marijuana is a Major Cause of Highway Accidents: By Lynn Zimmer, Associate Professor of
     Sociology, Queens College, & John P Morgan, Professor of Pharmacology, City University of New York
     Medical School: The Lindesmith Center, 1995 The detrimental impact of alcohol on highway safety has been
     well documented. Marijuana's opponents claim that it, too, causes significant impairment and that any
     increase in use will lead to increased highway accidents and fatalities. THE FACTS In high doses marijuana
     probably produces driving impairment in most people. However, there is no evidence that marijuana, in
     current consumption patters, contributes substantially to the rate of vehicular accidents in America. A
     number of studies have looked for evidence of drugs in blood or urine of drivers involved in fatal crashes. All
     have found alcohol present in 50 percent or more. Marijuana has been found much less often. Furthermore,
     in the majority of cases where marijuana has been detected, alcohol has been detected as well. (*1) For
     example, a recent study sponsored by the US National Highway Traffic Safety Administration (NHTSA)
     involving analysis of nearly 2000 fatal accident cases, found 6.7 percent of drivers positive for marijuana. In
     more than two-thirds of those, alcohol was present and may have been the primary contributor to the fatal
     outcome (*2) To accurately access marijuana's contribution to fatal crashed, the positive rate among
     deceased drivers would have to be compared to the positive rate from a random sample of drivers not
     involved in fatal accidents. Since the rate of the past-month marijuana use for Americans above the legal
     driving age is about 12 percent, on any given day a substantial proportion of all drivers would test positive,
     particularly since marijuana's metabolites remain in blood and urine long after its psychoactive effects are
     finished. A recent study found that one-third of those stopped for "bad driving" between the hours of 7 p.m
     and 2 a.m - mostly young males - tested positive for marijuana only. (*3). To be meaningful, these test results
     would have to be compared to those from a matched control group of drivers. A number of driving simulator
     studies have shown that marijuana does not produce the kind of psychomotor impairment evident with
     moderate doses of alcohol (*4). In fact, in a recent NHTSA study, the only statistically significant outcome
     associated with marijuana was speed reduction. (*5) A recent study of actual driving ability under the
     influence of cannabis - employing the same protocol used to test impairment-potential of medicinal drugs -
     evaluated the impact of placebo and three active THC doses in three driving trials, including one in
     high-density urban traffic. Dose-related impairment was observed in drivers' ability to maintain steady lateral
     position. However, even with the highest dose of THC, impairment was relatively minor - similar to that
     observed with blood-alcohol concentrations between 0.03 and 0.07 percent and many legal medications.
     Drivers under the influence of marijuana also tended to drive more slowly and approach other cars more
     cautiously While recognising some limitations of this study, the authors conclude that "THC is not a
     profoundly impairing drug." (*6) Refs: *1 : McBay AJ and Owens SM., "Marijuana and Driving", pp 257-63
     in L.S.Harris (ed) Problems of Drug Dependence 1980, Washington, DC: U.S. Government Printing Office
     (1981): Teale, JD et al., "The Incidmnece of Cannabinoids in Fatally Impaired Drivers: An Investigation by
     Radioimmunoassay and High Pressure Liquid Chromatography," Journal of the Forensic Science Society 17:
     177-83 (1978) *2 Terhune, KW et al., The Incidence and Role of Drugs in Fatally Injured Drivers,
     Washington DC: Department of Transportation (1994) *3 Brookoff, D et al., "Testing Reckless Drivers for
     Cocaine and Marijuana", New England Journal of Medicine 331: 518-22 (1994) *4 Kv'alseth, TO, "Effects
     of Marijuana on Human Reaction Time and Motor Control", Perceptual and Motor Skills 45: 935-39 (1977):
     Hansteen, RW, et al, "Effects of Cannabis and Alcohol on Automobile Driving and Psychomotor Tracking,"
     Annals of New York Academy of Science 282: 240-56 (1976): Moskowitz, H et al., " Marijuana: Effects on
     Simulated Driving Performance," Accident Analysis and Prevention 8: 45-50 (1976); Moskowitz H et al.,
     "Visual Search Behaviour While Viewing Driving Scenes Under the Influence of Alcohol and Marijuana",
     Human Factors 18: 417-31 (1976) *5 Stein, AC et al., A Simulator Study of the Combined Effects of Alcohol
     and Marijuana on Driving Behavior-Phase II, Washington DC: Department of Transportation (1983)
     Cannabis makes you a better driver - more evidence Source: 1 The AGE 21 October 1998 pA5; 2.
     CANBERRA TIMES 21 October 1998 p4 The largest study ever done linking road accidents with drugs and
     alcohol has found drivers with cannabis in their blood were no more at risk than those who were drug-free. In
     fact, the findings by a pharmacology team from the University of Adelaide and Transport SA showed drivers
     who had smoked marijuana were marginally less likely to have an accident than those who were drug-free. A
     study spokesman, Dr Jason White, said the difference was not great enough to be statistically significant but
     could be explained by anecdotal evidence that marijuana smokers were more cautious and drove more slowly
     because of altered time perception. The study of 2,500 accidents, which matched the blood alcohol levels of
     injured drivers with details from police reports, found drug-free drivers caused the accidents in 53.5 per cent
     of cases. Injured drivers with a blood-alcohol concentration of more than 0.05 per cent were culpable in
     nearly 90 per cent of accidents they were involved in. Drivers with cannabis in their blood were less likely to
     cause an accident, with a culpability rate of 50.6 per cent. The study has policy implications for those who
     argue drug detection should be a new focus for road safety. Dr White said the study showed the importance
     of concentrating efforts on alcohol rather than other drugs. This information was posted by the library of The
     Alcohol and other Drugs Council of Australia (ADCA). Requests for copies of newsclips can be directed to
     the library by phone 02 62811002, fax 02 6282 7364 or e-mail library@adca.org.au. To subscribe to this
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