How Cannabis Use Affects Driving

Increase in cannabis use raises concerns that users may be taking the wheel. Research indicates the effects of cannabis on driving are more complex than many assume.

Not surprisingly, impairment increases with higher doses. Tetrahydrocannabinol (THC), the psychoactive chemical in cannabis, reduces a driver’s ability to keep in the centre of the lane, to maintain a constant following distance, and to make quick decisions about manoeuvres such as passing. Faced with an unexpected event, a cannabis-impaired driver may find it difficult to react quickly.

Alcohol causes more impairment than cannabis and carries a demonstrably higher crash risk. Drivers under the influence of cannabis are acutely aware of their impairment. They consciously try to drive more cautiously, for example by slowing down, focusing their attention and avoiding risks. Drinking drivers show more risk taking and aggression in their driving, have no insight into their impairment, and do not try to compensate.

Some prescribed medications appear to make drivers more collision-prone than cannabis. A 2004 UK study on the medicinal use of cannabis concludes that impairment of driving ability from cannabis is within the range of (or lower than) that produced by medications commonly used for similar conditions. For example, benzodiazepene type medications commonly prescribed to seniors increase crash risk. One study found a five-fold higher risk in people who had used benzodiazepenes in the past three months.

Surprisingly, there seems to be little evidence that drivers who have used cannabis on its own are more likely to cause crashes than drug-free drivers. However, this does not mean it’s safe to mix cannabis with driving.

Only fatalities and serious injuries have been analyzed in the available research. To what extent cannabis is a factor in less serious collisions still needs to be investigated.

THC can be detected in blood or urine for days after smoking. Surveys that establish recent use of cannabis show that THC-positive drivers, especially at higher doses, are three to seven times more likely to be responsible for their crash than drivers who had not used drugs or alcohol. In other words, recent use of cannabis may increase crash risk, while past use of cannabis does not.

Few road fatalities test positive for THC alone. Most often, it is found in combination with alcohol. Controlled studies show this combination produces severe impairment. Driver casualty statistics confirm that using cannabis with alcohol dramatically increases crash risk.