Cannabis Use and Driving: The Effects and Difficulty of Testing

3rd January 2020

As an increasing number of jurisdictions opt to legalise Cannabis, the question of testing for impairment while driving has emerged at the forefront of concerns. Although this is not a new issue, tech companies and investors are pushing harder than ever before to develop suitable technology to aid in minimising the risks involved with driving under the influence of Cannabis. 

When a driver has been under the influence of alcohol, it is relatively simple for the police to detect, and hence prove impairment with the use of a breathalyser. Alcohol breathalysers have been around since 1954, and are standard issue among police officers. However, the same cannot be said for Cannabis. One of the major concerns among critics and opponents of Cannabis legalisation is the possible impact on road safety.

Alcohol Vs Cannabis

Although there is still little extensive research in the area, it is widely accepted that Cannabis use can impair the user’s ability to drive safely. It has been shown that the active compounds in Cannabis can affect response times and general motor performance.

Nevertheless, it is unsuitable to simply detect Cannabis use in the same way a breathalyser detects alcohol levels, as the way they impair the body differs. For example, the detection of cannabinoids does not necessarily guarantee impairment.

Extensive research studies have determined an acceptable limit to alcohol concentration in the blood, before the average person may become impaired. As alcohol leaves the body relatively quickly, it is accepted that alcohol levels can be effectively used to help determine impairment.

In a May 2019 report on Cannabis use and driving, Congressional Research Service transportation analyst, David Randall Peterman wrote:

“Based on current knowledge and enforcement capabilities, it is not possible to articulate a similarly simple level or rate of marijuana consumption and a corresponding effect on driving ability.”

In contrast, cannabinoids remain in the body for much longer. Evidence of Cannabis use may be detected for hours, days, and even weeks, after consumption. Yet, impairment caused by Cannabis usually lasts no longer than a matter of hours. This contrast highlights the need for an alternative method of detection.

The Effects of Cannabis on Road Safety

Cannabis being the most frequently used drug (after alcohol). Despite this, little evidence has been collected to demonstrate its effect on the risk of road accidents. This is partly due to the almost international prohibition of Cannabis over the last century.

Studies that have focused on this area of risk have also yielded contrasting results, with some suggesting little or no increased risk to road safety. However, others have shown that, in a laboratory setting, marijuana use can measurably affect a driver’s performance.

Although this may sound positive for supporters of Cannabis legalisation, it is not so straight-forward. Many of these studies have faced skepticism over whether they are representative of real-life examples. It cannot be determined for sure whether these findings may translate into an increased risk of a road accident.

Despite a lack of evidence demonstrating the effects of Cannabis on the ability to drive, some effects have been observed. For example, a decline in reaction times. Yet, studies have found that subjects under the influence of marijuana tended to alter their driving to compensate for their possible impairment.

This most often involved driving under the speed limit and taking fewer risks than they would when they were not under the influence of marijuana. In comparison, subjects under the influence of alcohol tended to drive over the speed limit and take more risks.

Detection Technology

It is accepted that identifying an acceptable level of THC can be more complicated than alcohol. However, some companies believe they are well on the way to creating the technology needed for accurate and suitable detection.

THC Breathalysers

In July 2019, the University of California found that THC could be detected in a subject’s breath for 2-3 hours after smoking Cannabis. The study also claimed that during this initial period, detected THC levels in breath correlates with levels in the blood. The study was sponsored by Hound Labs, which has raised $65 million to develop a dual alcohol and THC breathalyser.

However, this measurement is not designed to be a definitive method of detecting Cannabis impairment. Instead, it is designed to provide more information to the relevant authorities.

Hound Labs founder, Mike Lynn, said:

“We’re measuring THC in breath where it lasts a very short period of time, providing objective data about THC in breath to law enforcement and employers to use in conjunction with other information they have gathered”.


Another company has approached the issue from another angle, by focussing exclusively on the level of impairment. DRUIDapp, Inc. has developed a mobile app that can detect physical impairment in the user. DRUID (DRiving Under the Influence of Drugs) engages the user in four activities to measure potential impairment.

The app is designed to determine whether a person’s drug consumption (including alcohol) may affect their ability to drive safely.

The test consists of four tasks relating to reaction time, decision-making ability, hand-eye coordination, time estimation, and balance. All of these areas are known to relate to a person’s ability to drive. Three of the four tasks also involve the user to divide their attention. After completion, the user is given a score between 0-100. The higher your score, the less fit you are for driving.

The app is being employed by a number of Cannabis investigators, including several US universities. Researchers from the Johns Hopkins University Medical Institute examined the correlation between dosage-controlled Cannabis administration and the DRUID impairment scores. They found that the app was able to determine impairment resulting from different levels of Cannabis consumption.

The DRUID app is an innovative approach to help determine whether Cannabis (or any other drug) use may affect the user’s driving. It is an accessible and potentially helpful tool that can be used by anyone. The app is available through the App Store and Google Play for Apple and Android devices for 99p.

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