Smoking cannabis has been associated with an increased risk of a crash, but the effects of CBD on driving have not been examined to such an extent so far.
The study was conducted on 26 healthy occasional users of cannabis between May 20th, 2019, and March 27th, 2020. The users were recruited via advertisement, social media, and word of mouth. They were between 20 and 50 years old, with a body mass index between 20 and 28. All of them smoked cannabis less than two times per week in the 12 months before the study and more than 10 times in their lifetime. They all had a valid driver’s license with at least two years’ driving experience and were driving more than 2.000 km per year.
Before the test drives, the participants vaporized THC-dominant, CBD-dominant, THC/CBD-equivalent, and placebo cannabis. THC and CBD doses were 13.75 mg.
The on-road test
The on-road test was 100 km long and started at 40 minutes and 240 minutes after cannabis consumption. The test ran for approximately 60 minutes, with the participants driving a specially instrumented vehicle over a highway circuit while maintaining a constant speed (95 km/h) and a steady position in the slower traffic lane. Participants were accompanied by a licensed driving instructor who had access to dual vehicle controls.
The main indicator of the effects in this study was the standard deviation of lateral position (SDLP). The SDLP is an index of lane weaving, swerving, and overcorrecting that is a validated measure of alcohol- and drug-induced driving impairment. Apart from driving performance, the study investigated the cognitive function and subjective experiences of the drivers.
Lateral position, which is the distance between the vehicle and the lane boundary to the left of the vehicle, was recorded by a camera mounted onto the roof of the vehicle and sampled continuously at 4 Hz. Measurements of lateral position over the time of the driving test were averaged to yield the mean lateral position, and the standard deviation was calculated to determine the mean SDLP. Larger numbers indicate reduced stability. A 2.4-cm drug vs placebo increase in SDLP is typical of a driver with a blood alcohol concentration (BAC) of 0.05% and is thought to indicate the lower limit of clinically relevant driving impairment.
At 40 to 100 minutes following vaporized consumption, the SDLP was significantly increased by THC-dominant cannabis (+2.33 cm) and THC/CBD-equivalent cannabis (+2.83 cm), but not CBD-dominant cannabis (−0.05 cm), relative to placebo. The SDLP was 18.21 cm for CBD-dominant cannabis, 20.59 cm for THC-dominant cannabis, 21.09 cm for THC/CBD-equivalent cannabis, and 18.26 cm for placebo. At 240 to 300 minutes, the SDLP was 19.03 cm for CBD-dominant cannabis, 20.59 cm for THC-dominant cannabis, 19.88 cm for THC/CBD-equivalent cannabis, and 19.37 cm for placebo. Compared with placebo, SDLP with THC-dominant and THC/CBD-equivalent cannabis was significantly greater at 40 to 100 minutes but not 240 to 300 minutes after consumption, and there were no significant differences between CBD-dominant cannabis and placebo.
Out of 188 test drives, 16 (8.5%) were terminated due to safety concerns. Among 26 randomized participants, 22 (85%) completed all eight driving tests. One participant had a panic attack shortly after cannabis administration in the THC condition, leading to termination of that test day and withdrawal from the study. The rest of the 16 withdrawn participants were terminated by the driving instructor due to safety concerns. Seven of them were appearing heavily fatigued while driving.
Effects of CBD on driving – limitations of the study
This study has several limitations. Among the most important ones, the occasional cannabis use in participants might have produced partial tolerance to the impairing effects of THC. Also, the effects of CBD on driving are not investigated fully because only one dose of CBD and a single 1:1 ratio of CBD and THC were tested. The CBD dose used was lower than that used in clinical practice for conditions such as pediatric epilepsy in which oral administration of CBD oils at doses of approximately 10 to 20 mg/kg is common.
Furthermore, retail CBD products in North America and other regions are not strictly regulated and the actual CBD content may be unknown or misrepresented. This study was limited to a sample of young drivers with similar driving experience. The degree of driving impairment may differ as a function of driving experience as well as experience with cannabis and the driving task.