Cannabis and Driving, the Risks

Liz Filmer
23 Feb 2024

Because of how THC is processed by the body it is difficult to measure its impact on our cognitive abilities. Although it is widely accepted that it is not safe to drink and drive many people believe that cannabis doesn’t affect them in the same way as alcohol and so it is safer to drive when stoned.

Blood alcohol levels and impairment typically rise in balance with each other and breathalysers are a swift, reliable and non-invasive way to tell if someone is drunk.

Testing for stoned drivers, however, is not as straightforward. Some law enforcement areas have set criteria for what comprises cannabis impairment, (for example, five nanograms of THC per millilitre of blood)

As it stands no linear correlation can be found between the levels of THC concentration in the blood and driver impairment. Accurate testing still requires a blood or saliva sample.

To make matters harder to resolve, THC remains in the bloodstream for longer periods of time, a matter of weeks – so in comparison with alcohol, its existence within blood samples reveals little about a driver’s condition as there is no foolproof way to tell how long the THC has been present in the body. These intricacies have made it challenging to estimate what sort of a hazard THC poses to drivers and road users. As a result, it is common for some areas where cannabis is legal to have a predetermined legal limit for the concentration of THC in the blood, regardless of whether the driver appears impaired or not.

Data from a 2017 report carried out by the US National Highway Traffic Safety Administration, indicates that whilst cannabis can impair driving the odds of being implicated in a crash directly after using cannabis are still significantly lower than those same odds when alcohol is involved.

The theoretical explanation is that alcohol has an effect on drivers where it makes them more aggressive and prone to accidents, whilst drivers who choose to get high driving are instead more likely to drive too slowly. Several investigations cited by the aforementioned 2017 NHTSA report suggest that this is largely correct.

So whilst it’s typically considered illegal to drive while high, it’s still not apparent if driving whilst stoned is as serious a problem as drunk driving, public safety concerns demand that locations with legal cannabis legislation think carefully about the problem and take some action.

Before legalisation in Canada, the Dräger DrugTest 5000 was approved as a roadside spit tester for THC. However, there have been questions surrounding its accuracy as it is understood to have some inaccuracies which have resulted in the device delivering both false positives and false negatives. Further still, when it does measure correctly, it can’t assess the person's capability to drive. Another flaw was that the machine works only between 4C and 40C, rendering it useless during Canada’s long, cold winters. Since this time a second THC saliva test device (the Abbot soToxa) has been approved for use. 

Interestingly, Canadian federal law enforcement data from six months after legalisation showed that there had been no excessive rise in the number of charges for driving whilst under the influence of cannabis.

This is not to say, however, that driving whilst high does not happen. A survey by California delivery service Eaze examined the movements of regular cannabis users

64% of cannabis users self-report using their cars within four hours of consuming with 20% confessing to driving within an hour after ingesting. When asked there appeared to be no consensus amongst those surveyed about consumers about how much cannabis it was safe to take before driving. There was an admission however that the surveyed participants were less likely to drive with a child in the car whilst stoned.

Of those adults who consumed and drove, 77% believed that it did not affect their driving whilst 16% believed it improved their driving. Male and younger drivers were found to be more likely to drive within eight hours of consuming.

Until there is a more evidence-based solution for assessing stoned drivers. driving is another aspect of cannabis legalisation that remains in a grey area although most people would agree that it is best to avoid driving at all whilst under the influence.

More on this topic from Soft Secrets:


CBD Does not Impair Driving Ability

Liz Filmer