In the lead up to the legalisation of recreational cannabis in Canada and the ongoing legalisation in some US states, opposing parties continued to warn of the resulting increase in road traffic accidents. However, following legalisation, a number of studies have aimed to assess whether this is actually the case.

Cannabis is the most commonly used drug in the world. It is also widely assumed that the age group that is most likely to use cannabis is the same as those that cause the most road traffic accidents. Although, cannabis, like alcohol, does impair several driving-related skills, according to a study published in The American Journal on Addictions it’s more of a dose-related question.

The impact of cannabis may vary from person to person. For example, it depends on whether the consumer is an experienced user or not; an individual’s tolerance, and how much THC – the main psychoactive compound- is present in the cannabis will affect the level of impairment each person experiences.

Driving under the influence of cannabis

While some found that cannabis users are more likely to drive slower, compensate more effectively in dangerous situations and increase following distance, driving under its influence does appear to have its risks.

One study found that “marijuana use increases reaction time and the number of incorrect responses to emergencies.”

Others have found that drivers had not monitored the speedometer effectively and that cannabis impairment appeared to cause “increased decision time when passing, increased time needed to brake when a light suddenly changes, and increased time to respond to a changing light or sudden sound.”

As previously mentioned, although there are people with different background and consuming history, studies agree that the higher THC is in blood, the greater the driving impairment.

Driving under the influence of alcohol

Driving under the influence of alcohol is, as studies and available datasets show, considered a greater risk than driving while “high”. Alcohol does impair pursuit tracking, divided attention, signal detection, hazard perception, reaction time, attention, concentration, and hand-eye coordination.

While cannabis consumers tend to overestimate the risks on the roads, drunk people do the exact opposite. Even at lower alcohol doses, drivers are more likely to underestimate the risks and overtake in tight situations. In more than a half dozen studies cited that subjects who had consumed alcohol consistently drove faster.

According to a research made in France, drivers under the influence of alcohol are 17.8 times (12.1-26.1) more likely to be responsible for a fatal accident while the same number with regards to marijuana is 1.65 (1.16-2.34).

However, the real danger would appear to be of mixing alcohol and cannabis while driving. Studies found that “unexpected events are still difficult to handle under the influence of marijuana, however, the combination of low-dose alcohol and low-dose cannabis causes much more impairment than either drug used alone.”

Terhune and et al. found that of 1,882 motor vehicle deaths, according to their odds ratio (OR) 0.7 for cannabis use, 7.4 for alcohol use, and 8.4 for cannabis and alcohol use combined were responsible.

Does cannabis legalisation lead to an increase in road traffic accidents?

Although one study found that traffic deaths in three US states rose for a limited period of time after legalisation, the same study also showed that the numbers fell after three months. Despite research still being in its infancy, it would appear that cannabis legalisation does not lead to an increase in road traffic accidents.

A study found that shortly after local governments had legalised the recreational use of cannabis in three American states, the number of road traffic accidents rose by an average of one additional traffic fatality per million residents. However, this turned out to only be a temporary change.

These numbers mirrored the neighbouring jurisdictions during the examined period of time. Therefore, while driving under the influence of cannabis may increase the risk of being involved in a road traffic accident, it appears that cannabis legalisation does not lead to large or sustained increases in traffic accidents.

Furthermore, according to a research published in The Journal of Law and Economics, scientists found “a noticeable drop in traffic fatalities following legalisation.”