Regular cannabis users show no impairment in driving performance after abstaining from the drug for at least 48 hours, a new study has shown.
The study, published in the journal Psychopharmacology by researchers from the Centre for Medicinal Cannabis Research at the University of California San Diego, involved 191 cannabis users and examined their driving abilities using sophisticated simulator tests.
The research comprised two substudies: the first examined driving performance in moderate to heavy cannabis users after 48 hours of abstinence, whilst the second compared the driving performance of the most frequent cannabis users against a control group of non-users.
Participants in the first study were aged between 21 and 55 years old, required to have a valid driving license, and had used cannabis at least 4 times in the past 30 days. They had no previous serious mental health diagnoses and had to be prepared to abstain from cannabis use for at least 48 hours. Nearly half (48.7%) used cannabis more than 4 times a week, and 51.3% used it less than 4 times a week; the group’s mean usage over 30 days was 16.7 days. The median yearly miles driven by participants was 8,730 miles.
All participants completed a 25-minute driving test using a Console Driving Simulator System (CDS), a device that allows researchers to measure driving ability. The simulation was set in both urban and rural locations, and participants were faced with traffic challenges, including taking a left turn across traffic, crash avoidance, safely following the car in front, and driving at varying speeds.
“These variables were aggregated into the CDS (similar to an overall z score), by combining multiple measures providing a stable global measure of driving performance which is not dependent on performance on any single component of the driving task,” the researchers said.
When participants’ results were compared to standardised CDS scores, researchers found no significant differences.
In the second study, researchers compared 18 of the most frequent cannabis users from the first study against 12 non cannabis users. Frequent users were defined as participants who consumed cannabis on at least 28 of the previous 30 days.
Participants in the non-using group were aged 21 to 55 years, had driven more than 1000 miles in the past 12 months, hadn’t used cannabis in the past 12 months, and drank alcohol on less than 15 days of the previous 30.
The analysis “revealed no significant differences between the comparison group and frequent cannabis users on the CDS, nor on any of the subtests comprising the CDS or mean driving speed,” researchers reported.
As more people across the world are using cannabis medically and recreationally, and countries begin to change their stance on the plant, scientists and lawmakers are required more than ever before to thoroughly investigate the effects it can have on the body and on our lives.
How cannabis affects driving performance remains a contentious topic, with many conflicting opinions. What is unifying is how justice systems and employers associate cannabis use, and having quantities of cannabis in the body after use, with dangerous, and or poor performance, either when driving or at work.
In the UK, police detection of even small amounts of cannabis in blood typically results in a 12-month driving ban. Employers can also discipline their staff based on the results of drug tests, citing workplace safety as a concern.
However, this study adds to growing evidence questioning the assumption that cannabis automatically impairs driving ability.
Previous research, published in the journal JAMA, investigated the link between driving skill and cannabis use in older adults. Regular users of cannabis aged between 60 and 79 were tasked with completing two CDS tests, one while under the influence of cannabis, and the other while sober. The results showed no difference in test result scores.
Another recent study published in the Journal of Safety Research found that prescribed cannabis use before driving did not affect patients’ ability to drive safely.
While a 2023 study using both simulator and real-car tests, plus field sobriety assessments, similarly found no correlation between cannabis use and driving ability.
Researchers from the latest study acknowledged that larger studies are needed, but argued their findings highlight the need for policymakers to reexamine cannabis driving laws.
“The current study showed no evidence of a dose-effect relationship between simulated driving performance following a brief abstinence period and critical variables such as cannabis use intensity within the last 6 months, age of use onset, length of abstinence periods, or baseline blood cannabinoid concentrations in a cohort of regular cannabis users,” they concluded.
“The current study also shows no evidence of short-term residual effects on simulated driving performance when comparing frequent cannabis users to a healthy nonusing comparison group. Results of this study have implications for how future policy might weigh different pieces of evidence in the absence of objective confirmation of acute cannabis intoxication, such as one’s cannabis use history or residual blood THC, in everyday determinations of impaired driving.”
