Epidemiology findings: driving and cannabis

To have a clear understanding of the extent to which drivers around the world are driving under the influence of cannabis, some data had been collected from roadside surveys that shed some light into the extent of cannabis impaired driving worldwide.

In Europe, data shows that though alcohol is clearly the most common drug known to drivers, cannabinoids are confirmed to be present in over 2.4% drivers in Finland and 57% in Switzerland. Data were also collected on the prevalence of cannabis in (European) drivers who are injured or killed. Cannabinoids have been found in 7.6% of injured drivers and 1.5% of drivers killed!

Driver performance studies

cheech and chong driving

Participants were all subjected to cannabis smoking at least once a week, different doses of cannabis were used in this study, ranging from Placebo (0.005% THC) to high dose (2.67% THC) of cannabis.

Their performances were later measured using a driving simulator, a hazard perception task, and a compensatory tracking task. The driving simulator used involve three components to the simulator drive. The first component was a motorway drive which consisted of a 3-lane road. Other vehicles on the road were programmed in a way that their actions will require an immediate response, such as a car pulling out in front or braking suddenly.

The second component of the drive involved driving a ‘figure of eight’ loop. Participants were asked to drive between 30 and 40 mph through two large loops with constantly changing curve radii. The participants were therefore required to make continuous steering corrections in order to stay in the middle of the lane.

The third component of the drive was a dual-carriageway with four intersections with traffic lights. The signal changed colour as the driver approached.

For the hazard perception task involved viewing video recordings of situations that would require the driver to take immediate action such as swerving or braking suddenly. The reaction time to detect hazards and the proportion of hazards detected were the dependent measures.

Finally, the compensatory tracking task involved tracking a moving circle on a computer screen with the mouse while responding to the changing colour of symbols in the four corners of the screen at the same time. Using visual analogues their various moods were recorded.

Cannabis effect on driving skills

Cannabis was seen not to have dramatic influence on their driving performance, really? Yes! Basically, drivers mean speed was seen to reduce, a sign that was interpreted as drivers being aware of their impairment, and adjusting their performance to make the task easier, thereby compensating for the effects of cannabis by slowing down and not taking risk (like overtaking). However, this compensation is not possible when the driver encounters unexpected events and/or when the driver is placed in situations requiring increased mental load or continuous attention.

driving high

How much can cannabis affect my driving?

Each participant were given cannabis of different dose to smoke, 11.5 mg of THC for the low dose and 17.9mg of THC for the high dose. After some minutes, blood levels of THC (in ng/ml) at 10 and 30 minutes after smoking were taken. For the high dose 478ng/ml and 105ng/ml were recorded, for the low dose were 370ng/ml and 102ng/ml. To avoid any form of error, the 10 minutes time point was selected because it has been suggested as the time that THC concentrations are at their peak. While the blood levels for the high dose were considerably greater at 10 minutes after smoking than for the low dose.

The above findings made a lot of sense when “stoned” drivers were analyzed, it was found out that the driver that consume  high dose of cannabis (THC) shows more of the cannabis effect than the low dose driver. Meaning, the concentration of “crack” smoked has a lot influenced to which how much of your driving is affected.

How long can I wait before driving after taking “crack”?

According to current recommendation from evidence-based Canadian Low Risk Cannabis Use Guidelines (LRCUG), it’s recommended that users of cannabis should categorically refrain from driving for at least 6 hours after using cannabis. But it should also be noted that depending on concentration, the user, and the specific cannabis product consumed, this wait time might be extended. Studies over the years have shown that those that drink (alcohol) with smoking of cannabis have increased impairment, so you might want to consider not driving after taking both.

Is driving stoned legal?

With the legalization of cannabis in Canada, one would be anxious to know if it’s a crime to get stoned while driving. As any police officer would say, it is already illegal to drive when impaired by any drug (cannabis inclusive). With any reasonable suspicion of impairment in a driver such as bloodshot eyes, slurred speech and odor of alcoholic beverages in breath, an officer can further do a test either through saliva test or with the use of field sobriety tests on the drivers. So, if caught be ready for the punishment and fines that follows!


The negative impact of cannabis use upon driving performance is dose-related, increasing with increased dosage of cannabis. Unlike alcohol use, cannabis use does not predispose to aggressive or violent behavior, which can affect driving ability. Use of cannabis and alcohol in combination creates an additive risk of impairment and serious motor vehicle accident.

Despite the concern, studies show that cannabis impairs a person’s ability to drive. Furthermore, though there are variations in the acceptance of using cannabis, it is illegal to drive under the influence of drugs. Aside that, lots of accidents have been attributed to driver’s consumption of drugs while driving. In order words do not drive when you are ‘cracked’ up!