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.
In 1985, Moskowitz published a
remarkable synthesis of studies on the psychomotor and cognitive effects of
cannabis.[1] 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] 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]
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
|
|
Smoked
< 9mg
9 – 18 mg
³ 18 mg
Total
Oral
< 9mg
9 – 18 mg
³ 18 mg
Total
|
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.
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]
|
|
Reference / environment
|
Subject / Dose / Protocol
|
Tasks
|
Measurements
|
Results
|
|
Simulator
Liguori et al., 1998
Sexton et al., 2000
|
10 users
Placebo
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
Placebo
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
Placebo
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
Placebo
100
Test: 30 mn after
16 users
Placebo
Alcohol level: 0.5 g/l
18 users
THC: 100, 200
Alcohol: 0.4 g/l
Preparation:
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
Ditto
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
|
