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Major Studies of Drugs and Drug Policy
Canadian Senate Special Committee on Illegal Drugs
Volume I - General Orientation

Chapter 8 - Driving Under the Influence of Cannabis

Experimental studies


Epidemiological studies indicate a relatively high level of driving under the influence of cannabis, between 5% to 12% of drivers, mostly among young men. At the same time, neither these studies nor the responsibility/risk analyses reach clear conclusions concerning the role of cannabis in dangerous driving. Hence the interest in studies on how cannabis affects driving ability and driving itself. Studies on the psychomotor and cognitive skills needed to drive vehicles have measured factors such as: motor coordination, reaction time, attention, visual attention and deductive reasoning. There are two types of studies on driving: simulated studies and field studies, whether on a track, in the city or on a highway. Most studies focus on single doses for recreational users. They use control group protocols and cross-linked protocols, including placebos and comparisons with alcohol. However, they are limited by the fact that they mainly measure the acute effects of single doses, making it difficult to determine whether more experienced users would react in the same way. The following sections examine both types of study.


Non-driving activities

In 1985, Moskowitz published a remarkable synthesis of studies on the psychomotor and cognitive effects of cannabis.[1][25] In this synthesis, he examined motor coordination, reaction time, tracking and sensory functions. The author observed the following:

··                motor coordination, measured by hand stability, body balance and movement accuracy was significantly affected. However, the application of these results to driving a car is limited, except in driving situations that require considerable coordination, such as emergency situations. The limits in terms of dose and number of subjects tested (between 8 and 16) also need to be noted

··                reaction time was not significantly changed: “There are a sufficient number of experiments involving both simple and complex reaction time situations to leave us relatively well assured that neither the speed of initial detection nor the speed of responding are, per se, impaired by marihuana. Rather, when marihuana produces a reaction time increase, there is some dimension of the information processing task which the subject must execute which bears the brunt of the experiment.”[2][26] Attention rather than reaction time was affected by marijuana use

··                straight line: this dimension was particularly sensitive to the effects of marijuana, and the vast majority of studies showed a significant reduction in the ability to go in a straight line or correct deviations from the line

··                the sensory functions (hearing and visual) are often affected, but the studies did not yield precise results concerning the distinction between simple tasks and complex tasks.


Ramaekers et al. (2002), reported a meta-analysis on 87 controlled laboratory studies on the psychomotor effects of cannabis conducted by Berghaus et al. (1998). These authors found that the number of psychomotor functions linked to driving (following, reaction time, perception, hand-eye coordination, body balance, signal detection and divided and continuous attention) affected by THC reached a maximum during the first hour after smoking, and one to two hours after oral ingestion. The maximum figures were comparable to those obtained with an alcohol concentration equivalent to > 0.05 g/dl. The number of functions affected reached zero after three to four hours, and only higher doses continued to have an effect. The studies surveyed also showed that THC concentration in the blood is highly correlated to psychomotor effects: a concentration of between 14 ng/ml and 60 ng/ml affected between 70% and 80% of tasks.[3][27]

The following table summarizes these data:



Deterioration of performance on psychomotor tests by dose,

time and method of ingestion

THC dose

Time (in hours)

        < 1                     1-2                      2-3                      3-4                        4-5              


Tests (n)  % affected

Tests (n)  % affected

Tests (n)  % affected

Tests (n)  % affected

Tests (n)  % affected


< 9mg

9 – 18 mg

³ 18 mg




< 9mg

9 – 18 mg

³ 18 mg




271271        61%

193193             53%

6464                 64%

528528             58%



3           33%

 3             0%

33                        0%

9            11%


3333                     36%

 48           38%

 28           36%

109109                   37%



 49           14%

 41           39%

 45           60%

135          37%


 10           30%

   8           38%

 10           40%

2828                     36%



 37            8%

 45          18%

 15          33%

 97          20%


 10            0%

   6            0%

 15          53%

 31          26%



 13            8%

 17           18%

 15           33%

 45           20%


 11            0%

   2            0%

   3           67%

 16           13%



  -                -

  -                -

 11           45%

 11           45%



More recently, after surveying the studies carried out in recent years, the reports prepared by INSERM and the International Scientific Conference on Cannabis reached largely similar conclusions: cannabis affects reaction time where choice is involved, road tracking, shared attention and continuous attention, as well as memory processes, but does not significantly affect simple reaction time or visual or eye-movement functions.


While driving

One of the weaknesses of the laboratory studies is the difficulty of relating psychomotor and cognitive tasks directly to driving. Several tests measured in these studies are short and relatively simple and do not necessarily reflect real situations. The advantage of simulated driving studies and field driving studies is that it brings the conditions closer to reality.

Most contemporary studies have similar characteristics: subjects have had a driver’s licence for at least three years. They are often regular cannabis users. The subjects receive either cannabis or a placebo in a double-blind situation that is very strictly timed to control the level of THC transmitted. In some instances, the experimenters also include comparisons with alcohol and an alcohol placebo. However, it is impossible to control how much subjects inhale and actually absorb. The cannabis prepared by the U.S. National Institute of Drug Abuse (NIDA) varies between 1.75% THC for low doses, 2.67% for moderate doses and 3.95% for strong doses. Converted into mg/kg of weight, the doses correspond to 100, 200 and 300 mg/kg, whereas the heavy dose usually preferred by regular users is generally 308 mg/kg. The subjects are familiarized with the equipment used and the tasks to be performed, and are accompanied by instructors on actual driving studies. Measurements include the standard deviation of lateral position in relation to the road, the control over longitudinal position (distance) in relation to the vehicle ahead, decision-making in emergencies, style of driving and risk taking.

The following table, adapted from INSERM data, summarizes a number of the more recent studies.



Effects of cannabis on car driving[4][28]

Reference / environment


Subject / Dose / Protocol





Liguori et al., 1998














Sexton et al., 2000






10 users


Cigarette 1.77% THC smoked in 5 mn

Cigarette 3.95% THC smoked in 5 mn

Test: 2 mn after

Duration: 1 hour








15 users


Grass, low dose 1.77% THC

Heavy dose: 2.67% THC

1 resin cigarette: 1.70% THC

Blood and saliva sample 10 mm after start

Test 30 mn

Duration: 25 mn







Avoid a barrier that suddenly appears by braking (55 to 60mph)







Judgment: maintain speed of 30mph on marked road and select widest lane at intersection


Highway section with vehicle ahead passing



Highway section with vehicle ahead braking


16.7 km of highway section



Left and right turns





Intersection with traffic lights, with 4 lane road








Total braking time




Lag time to take foot off accelerator and step on brake



Average speed Number of cones knocked over

Number of successful choices


Average reaction time



Average reaction time


Maximum, minimum and average speed


Standard deviation for perfect line




Response time in going through amber


Average waiting period at a point 10m from the stop line


↑ Slightly significant at 1.77 THC, slightly more at 3.95


No difference




No effect






↑ At low dose (high level of variability at heavy dose: ns)


↑ At low dose (ns)



↓ Average of 6mph at low and heavy dose


↑ Variation at heavy dose versus low dose or placebo


↓ At heavy dose




↓ At heavy dose (high level of variability: ns)

Actual driving

Robbe, 1998 study No. 1 Closed portion of highway (cannabis)





Study No. 2

Normal traffic on highway (cannabis)








Study No. 3

City driving (cannabis)






Study No. 3

City driving (alcohol)






Robbe, 1998

Highway driving (cannabis and alcohol)
















Lamers and Ramaekers, 2000

City driving (cannabis and alcohol)



24 users


100, 200 and 300

Test: 40 mm and 1 hour 40 mm after





16 users

same doses as study 1

Test: 45 mn after









16 users



Test: 30 mn after





16 users


Alcohol level: 0.5 g/l






18 users

THC: 100, 200

Alcohol: 0.4 g/l


Alcohol 0 + THC 0

Alcohol ) + THC 100

Alcohol 0 + THC 200

Alcohol 0.4 + THC 0

Alcohol 0.4 + THC 100

Alcohol 0.4 + THC 200

Alcohol plus cannabis 60 mn after

Tests between 9:00 p.m. and 11:15 p.m.






16 users

THC 100

Alcohol 0.5 g/l

4 preparations:

Alcohol 0 + THC 0

Alcohol 0.5 + THC 0

Alcohol 0 + THC 100

Alcohol 0.5 + THC 100

Tests: 15 mn after

Duration: 45 mm


Constant speed at 90km/hr and tracking over 22km







Tracking control (Ibid.) 64km, 50 mn


Following cars over 50m at variable speed (between 80 and 100km/h) over 16 km, 15 mn




City driving 17.5 km

Dense, moderate or light traffic














Tracking: speed at 100km and constant lateral position








Following: follow a vehicle over 50 m with speed varying by ± 15km/hr every 5mn


Driving in traffic




City driving 15 km




Visual search monitoring


Standard deviation of lateral position


Average lateral position deviation


Average speed and standard deviation


Same measurements


Average reaction time


Average distances and standard deviations



External observations


Internal observations: skill, manoeuvres, turns…


External observations


Internal observations: skills, manoeuvres, turns…


Standard deviation of lateral position









Reaction time





Average distances and standard deviations


Frequency of appropriate eye movements


Quality of driving







↑ Instability at all 3 doses


No effect



No effect



Same effects



↑ ns



Distance increased by 8, 6 and 2 m for 100, 200 and 300 THC


No significant change


No effect





No significant change


0.34 g/l alcohol level modifies control and manoeuvres


↑ Tracking variability; low alcohol alone, THC 100 alone; Moderate: THC 200

Heavy: alcohol 0.4 and THC two doses


↑ Reaction time for 0.4 alcohol and THC 200



↑ Variability in distance between cars in all cases


No effect with alcohol alone or cannabis alone


↓ Performance if alcohol + cannabis

No effect




It is interesting to recall that one of the first driving studies on the road was conducted for the Le Dain Commission.[5][29] In this study, on a closed track, 16 subjects were each given the 4 following preparations: placebo, marijuana 21 and marijuana 88 μg/kg THC and a dose of alcohol equivalent to BAC 0.07. The tests were conducted immediately after use and three hours later. The subjects were to complete six circuits of the track (1.8 km) with manoeuvres involving slowing down while going forward and backwards, maintaining a trajectory and weaving through cones. The alcohol and heavy dose of marijuana decreased driver performance in tests conducted immediately after use. At the heavy cannabis dose, drivers drove more slowly. On the second test, the differences were less clear.

When the results of this study are compared to those conducted more recently using much more sophisticated methods, it can be seen that the results are remarkably similar.[6][30] Thus the following was observed:

··                lateral control: this is the variable that is most sensitive to the effects of THC, but the effects are variable, depending on the dose and time; only heavy doses significantly affected lateral control over the vehicle. In comparison, alcohol has a greater effect on vehicle lateral control and speed (linked variables)

··                speed control: in almost all cases, the use of cannabis significantly decreases speed

··                risk-taking: in addition to decreasing speed, it is generally found that there is an increase in distance between vehicles among marijuana users, and less of a tendency to pass or attempt dangerous manoeuvres

··                decision time: this variable is particularly important in actual driving situations. The results do not appear to be very consistent. Smiley suggests that reaction time is unaffected when the subjects are told that they need to respond rapidly, whereas on the other hand, when the obstacles are completely unexpected, the subjects who used cannabis do not perform as well

··                combined effects of alcohol and cannabis: when the researchers checked the effects of the two substances, the combined effects of cannabis and alcohol were systematically greater than alcohol alone or, even more so, than cannabis alone.


Lastly, with low doses, subjects had the impression that their driving was not as good as observers felt it was, which was not necessarily the case with higher doses, where the perceptions of both the drivers and the observers agreed.



[1][25]  Moskowitz, H., (1985) “Marihuana and Driving.” Accid. Anal. Prev., 17 (4), pages 323-345.

[2][26]  Ibid., page 330.

[3][27]  Ramaekers J.G. et al. (2002), op. cit., page 77.

[4][28]  Table adapted from INSERM (2001) op. cit., pages 183-184.

[5][29]  See Hansteen, R.W, et al. (1976) “Effects of cannabis and alcohol on automobile driving and psychomotor tracking.” Annals of the New York Academy of Science, 282, pages 240-256.

[6][30]  See notably the survey of studies and the discussion in Smiley, A., (1999) “Marijuana: On-Road and Driving Simulator Studies” in Kalant, H. et al., (ed) The Health Effects of Cannabis. Toronto: Addiction Research Foundation, pp. 173 passim.

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