Blood THC levels don't reliably predict whether someone is too impaired to drive | Research

The takeaway

Standard legal THC blood limits failed to correctly identify impaired drivers nearly half the time in a simulated driving study.

Medical cannabis patients who drive, policymakers working on impaired driving laws, anyone concerned about roadside drug testing accuracy

46% of participants exceeded THC blood limits but showed no driving impairment

What the researchers found

At 30 minutes after vaporizing THC, 46% of participants who exceeded legal THC blood limits showed no measurable driving impairment. At 3.5 hours, 57% showed impairment despite having THC levels below legal limits.

Why it matters

Many jurisdictions base impaired driving laws on THC blood concentration thresholds. If those thresholds can't distinguish impaired from unimpaired drivers, innocent people may be penalized while genuinely impaired drivers go undetected.

The numbers in context

14 participants; 46% not impaired at 30 min despite exceeding THC limits; 57% impaired at 3.5 hours despite THC below limits; median blood THC at 3.5h was 1.0 ng/mL

How the study worked

14 infrequent cannabis users completed simulated driving tests at two timepoints under three conditions (THC-dominant, THC/CBD, placebo), with blood and oral fluid THC measured against various per se limits.

What this study cannot tell us

Very small sample (14 participants), all infrequent users, simulated rather than real-world driving, single dose tested.

How to read the evidence

Small randomized controlled trial with only 14 participants using simulated driving

When this study was published

Published in 2021. Driving impairment testing methods and per se limit policies continue to evolve.

The bigger picture

These findings challenge the scientific basis for per se THC driving laws used in multiple U.S. states, Canada, and other countries, highlighting the need for better impairment detection methods.

Questions still open

What alternative impairment detection methods (cognitive testing, app-based assessments) could replace or supplement blood THC limits? Do frequent cannabis users show a different relationship between THC levels and impairment?

Common questions

Can blood THC levels predict driving impairment?

This study found they cannot do so reliably. Nearly half of participants who exceeded legal THC limits showed no measurable driving impairment, while over half were impaired when their THC levels had dropped below the limits.

How long after using cannabis is driving affected?

In this study, impairment was more common at 3.5 hours post-inhalation than at 30 minutes, even though THC blood levels were lower at the later timepoint. The relationship between timing and impairment varies by individual.

Read the original research

The failings of per se limits to detect cannabis-induced driving impairment: Results from a simulated driving study.

Traffic injury prevention, 22(2), 102-107

View on PubMed ↗Publisher’s paper ↗

Citation

Arkell, Thomas R; Spindle, Tory R; Kevin, Richard C; Vandrey, Ryan; McGregor, Iain S. (2021). The failings of per se limits to detect cannabis-induced driving impairment: Results from a simulated driving study.. Traffic injury prevention, 22(2), 102-107. https://doi.org/10.1080/15389588.2020.1851685