A driving simulator study found cannabis impaired visual acuity, contrast sensitivity, and stereoacuity, with the visual impairments directly correlated with worse driving performance.
Traffic safety researchers, ophthalmologists, and cannabis users who drive.
First study linking cannabis visual impairment directly to worse driving (r=0.504)
What the researchers found
In 20 young drivers, smoking cannabis significantly worsened visual acuity, contrast sensitivity, and stereoacuity (depth perception). Driving performance deteriorated, particularly lane-keeping. Crucially, the visual impairments and driving impairments were significantly correlated (r=0.504 for contrast sensitivity at near distances), providing the first evidence that cannabis-induced visual effects directly contribute to driving impairment.
Why it matters
Previous cannabis-driving research focused on cognitive impairment. This is the first study showing that cannabis-induced visual changes themselves contribute to worse driving, adding a new dimension to understanding impairment.
The numbers in context
20 drivers; significant impairment in visual acuity, contrast sensitivity (p=0.004), near stereoacuity (p=0.013), and far stereoacuity; lane-keeping worsened; correlation r=0.504 between contrast sensitivity and driving.
How the study worked
Within-subjects study of 20 drivers and occasional cannabis users (mean age 23.3) evaluated at baseline and after smoking cannabis, assessing visual function and simulated driving performance.
What this study cannot tell us
Small sample (n=20); mostly male; single cannabis smoking session; no dose-response assessment; simulated rather than real-world driving; participants were occasional users, not naive or heavy users.
How to read the evidence
Preliminary: small within-subjects study; novel finding but needs replication in larger samples.
When this study was published
Published 2020.
The bigger picture
Cannabis impairs driving through multiple mechanisms: cognitive (attention, reaction time), motor (coordination), and now visual (acuity, depth perception). This multi-pathway impairment may explain why cannabis-impaired driving is more dangerous than some cognitive tests alone would predict.
Questions still open
- How long do the visual impairments last after cannabis use? Are regular users equally affected, or does tolerance develop for visual effects?
Common questions
Does cannabis affect vision?
Is the visual impairment connected to driving problems?
Read the original research
Effects of Smoking Cannabis on Visual Function and Driving Performance. A Driving-Simulator Based Study.
International journal of environmental research and public health, 17(23)
Citation
Ortiz-Peregrina, Sonia; Ortiz, Carolina; Castro-Torres, José J; Jiménez, José R; Anera, Rosario G. (2020). Effects of Smoking Cannabis on Visual Function and Driving Performance. A Driving-Simulator Based Study.. International journal of environmental research and public health, 17(23). https://doi.org/10.3390/ijerph17239033