New study shows cannabis can impair driving for more than five hours—long after users feel ready to drive

A new study published in the Journal of Psychopharmacology has found that cannabis use can impair key driving skills for up to five and a half hours after consumption—well beyond the commonly assumed three-hour window. The study also revealed that many users feel ready to drive long before their driving performance returns to normal.

The researchers were motivated by the rapid expansion of legal cannabis use across the United States and the rising number of car crashes involving drivers who test positive for tetrahydrocannabinol (THC), the primary psychoactive component of cannabis. In 2021 alone, over 52 million Americans reported using cannabis. Yet existing research has not kept pace. Most past studies have only assessed driving within a narrow three-hour window after cannabis use, leaving questions about how long impairments might actually last. In addition, current laws in many states rely on THC blood levels to infer impairment, even though earlier findings have suggested that these levels may not accurately reflect a person’s ability to drive safely.

“With cannabis being the second most widely used recreational drug after alcohol, understanding its impact on driving behaviors is critical for public safety. Previous research on cannabis-impaired driving often focused on limited metrics and short timeframes, leaving gaps in knowledge about the duration and scope of impairment. Our interest was driven by the need to explore these gaps systematically, using a robust experimental design to assess multiple driving behaviors over extended periods,” explained study author Shashwath A. Meda, a senior clinical research associate at the Institute of Living at Hartford Healthcare and staff scientist at Yale University.

The researchers designed a controlled laboratory experiment that could measure multiple aspects of driving performance across an extended period of time after cannabis use. They recruited 38 healthy adult participants between the ages of 18 and 40, all of whom had at least two years of recent highway driving experience and used cannabis at least once per week. Most were frequent users, and both male and female participants were included. To be eligible, participants had to be free of significant medical or psychiatric conditions and had to abstain from cannabis for at least 24 hours before testing.

The study used a randomized, double-blind, placebo-controlled design. Each participant completed three full-day sessions, during which they were given either a placebo, a low dose of cannabis, or a higher dose, delivered by a vaporizer. Participants inhaled the vapor in a standardized way to ensure equal dosage across the group.

“The doses used (5.9% and 13% THC) were lower than those commonly found in commercially available cannabis products, which may underestimate the degree of impairment seen in real-world settings,” Meda told PsyPost.

Over an eight-hour period following cannabis use, participants completed a series of simulated driving tasks at four different time points. The simulations included scenarios such as lane-keeping, car-following, and overtaking other vehicles. These tasks were designed to test different driving abilities, such as maintaining control of the vehicle, responding to changes in traffic, and making safe decisions in potentially risky situations. In total, 19 different measures of driving behavior were assessed. Participants also regularly reported how “high” they felt, how impaired they believed they were, and whether they would choose to drive in their current state.

The results showed clear and measurable impairments across several key areas of driving. In the lane-keeping task, participants showed fewer steering corrections—an indication of reduced attention and control—for up to 3.5 hours after the low dose and up to 5.5 hours after the higher dose.

In the car-following task, participants exhibited less consistent use of the gas pedal and slower reactions to the lead vehicle for up to three hours after using the higher dose. During the overtaking task, those who received the high dose chose narrower gaps between cars, spent more time in the oncoming lane, and had lower time-to-collision estimates—behaviors that could increase the risk of a crash.

“Cannabis-induced driving impairments can persist longer than previously assumed, with certain effects lasting up to 5 hours after consumption of higher THC doses (13%),” Meda explained. “This timeframe exceeds the typical 3-hour window explored in earlier studies.”

The researchers also found a disconnect between how participants felt and how they performed. While most subjects reported feeling impaired for only two to three hours, the driving impairments lasted longer. In fact, many participants said they were willing to drive just two hours after using cannabis, even though objective measures showed their performance was still affected. This mismatch between subjective judgment and actual ability suggests that people may be unaware of the risks they pose when driving under the influence.

“Approximately two-thirds of participants were willing to drive despite being aware of their impairment, highlighting a significant public safety concern,” Meda noted. “Self-perceived readiness to drive does not align with objective measures of driving performance.”

The researchers also tested whether the concentration of THC and its metabolites in the blood or oral fluid could predict how impaired someone was behind the wheel. The researchers found no consistent relationship between these biological markers and actual driving behavior. In other words, a person could have a high level of THC in their system and drive safely, or have a low level and still show clear impairments. This finding reinforces growing concerns that blood THC levels are unreliable indicators of impairment and may not be suitable for legal or forensic use.

“One surprising finding was the lack of correlation between blood THC levels and driving performance metrics,” Meda said. “Despite elevated THC concentrations in participants’ blood post-consumption, these levels did not reliably predict impairment across various driving tasks. This disconnect suggests that biological measures alone may be insufficient for assessing cannabis-related driving risks.”

While the study had several strengths—including its detailed examination of driving behaviors, its use of standardized doses, and its extended testing window—it also had some limitations. The sample size was modest and included few occasional users, females, or older adults, which could limit how broadly the findings apply. Additionally, Meda noted that “simulated driving conditions cannot fully replicate real-world complexities, such as unpredictable traffic scenarios or environmental variables.”

Looking ahead, the research team plans to expand this line of work by incorporating brain imaging to explore how cannabis affects brain function during driving and examine the effects of higher THC doses.

“The primary goal is to refine understanding of cannabis-induced driving impairments by exploring additional metrics and scenarios that capture real-world complexities,” Meda explained. “We also aim to uncover the effects of cannabis on relationships between driving measures and objective brain function responses using fMRI. Future studies could investigate tolerance effects among chronic users, dose-response relationships with higher THC concentrations, and alternative methods for assessing impairment beyond blood THC levels. Ultimately, this research aims to inform evidence-based public policies and develop tools for accurately identifying impaired drivers.”

“This study underscores the importance of educating cannabis users about their impaired decision-making abilities while under the influence. Public health campaigns should emphasize that self-perceived readiness to drive does not equate to actual safe driving capability. Additionally, policymakers should reconsider reliance on blood THC levels as indicators of impairment and explore multifaceted approaches integrating behavioral assessments.”

The study, “A randomized, placebo-controlled, double-blind, pilot study of cannabis-related driving impairment assessed by driving simulator and self-report,” was authored by Shashwath A. Meda, Michael C Stevens, Erwin R Boer, Brian Pittman, Ralitza Gueorguieva, Marilyn A Huestis, and Godfrey D Pearlson.