Driving Performance and Cannabis Users’ Perception of Safety – Review

Driving Performance and Cannabis Users’ Perception of Safety – Review

JANUARY 31 2022


By Dr Michael White – Adjunct Senior Fellow, School of Psychology, University of Adelaide, South Australia

Marcotte et al. (2022): Per se laws are unjust

One of the best cannabis-impairment studies that I have encountered (Marcotte et al., 2022) was published online this week, as an open-access article. You can read it here:
Driving Performance and Cannabis Users’ Perception of Safety: A Randomized Clinical Trial | Adolescent Medicine | JAMA Psychiatry | JAMA Network

This large-scale (n = 191 subjects) study has a very strong emphasis on ecological (‘real-world’) validity.

While there are many interesting findings, perhaps the most relevant for drug-driving policy is that only about half of the subjects were impaired after using cannabis to a level that gave them a normal ‘high’. So, testing positive for Tetrahydrocannabinol (THC) at a roadside drug testing (RDT) site, even immediately after using cannabis, is a very poor indication that a driver is impaired. Stand-alone cannabis-presence (per se) driving laws are therefore unjust.

A second interesting finding is that there was no dose-response relationship between blood concentration of THC and impairment. So, it is nonsense for a government authority to say that the THC cut-off they use is sufficiently high to guarantee that a driver is impaired. That is a lie that some road-safety authorities have promulgated, for example, in the Australian State of Victoria.

A third interesting finding is that any impairment lasts for a maximum of 4.5 hours. Road-safety authorities in Australia boast that a driver can test positive for THC for 24 hours after using cannabis. One consequence is that a medical user of cannabis, who used only in the evening before bedtime, would always be THC-positive during the next day, but would never be impaired while driving.

The study’s conclusions are reproduced below:

Conclusions: In a placebo-controlled parallel study of regular cannabis users smoking cannabis with different THC content ad libitum, there was statistically significant worsening on driving simulator performance in the THC group compared with the placebo group. The THC content of the cannabis and intensity of prior cannabis use were not associated with driving outcomes; participants self-titrated in a manner that yielded similar reductions in driving performance, despite achieving different THC blood concentrations. A lack of insight regarding driving impairments, particularly at 90 minutes, is of concern, given that users will likely self-evaluate when they feel safe to drive. Although performance was improving at 3.5 hours, recovery was not fully seen until 4.5 hours post-smoking. The fact that not all participants consuming THC met the criteria for impairment underscores the interindividual variability seen with the impairing effects of cannabis. The lack of relationship between blood THC concentration and driving performance raises questions about the validity of per se laws. When users control their own intake, one cannot infer the level of impairment based on the THC content of the product, the level of behavioral tolerance in the individual, or the blood THC concentration. Future research should address factors such as individual biologic differences, personal experience with cannabis, and cannabis administration methods in relation to driving impairment


Marcotte, T. D., Umlauf, A., Grelotti, D. J., Sones, E. G., Sobolesky, P. M., Smith, B. E., Hoffman, M. A., Hubbard, J. A., Severson, J., Huestis, M. A., Grant, I., & Fitzgerald, R. L. (2022). Driving performance and cannabis users’ perception of safety: A randomized clinical trial. JAMA Psychiatry. Published online January 26, 2022.