Drugs and Driving: Culpability and the Results From Simulator Trials By Dr Luke Downey

Posted on: June 8, 2019
Last Updated: May 26, 2021

Associate Professor Luke Downey is an NHMRC R.D. Wright Biomedical Career Development Fellow. He leads the Drugs and Driving Research Unit at Swinburne, and specializes in human psychopharmacology with a particular interest in how individual differences in emotional intelligence contribute to human behavior. In 2015, Dr. Downey held a visiting scholar position at Harvard Medical School.


Author Quotes:  

When you consider when people had a legal amount of alcohol in their system (less than 0.05% BAC), they were only 1.2x more likely to be responsible for the accident. But the thing to note is, they were 3x more likely to be involved in an accident!

So, does the presence [of drugs] and their culpability give us the whole story, give us the full causation? Probably not – so the other way we examine the effects of drugs and alcohol on driving is to conduct randomised controlled trials.

Surprisingly, regular cannabis users displayed more driving errors than ones that didn’t consume cannabis as often, so maybe there’s a bit of self confidence in that, but they’re still making more errors so it’s a bit of a concern there with your longer-term users.


Summary and slides: 

 In Part II of Prof Downey’s presentation on the impact of drugs on driving, he begins with an overview of how drug testing is carried out in Australia. 

Discussing how culpability is determined, he highlights how identifying mitigating categories might be a better way of determining the role of drugs in driving. 

He moves on to talk about whether we are testing for the right drugs, and how a number of people are now testing for both alcohol and drugs in their system. 

Prof Downey presents data from simulated trials for cannabis and alcohol which reveal regular cannabis users displayed more driving errors than non-regular cannabis users. 

Concluding this presentation, we take a look at detection measures and how a person’s scanning distribution is affected by different drugs and combinations. 

Take-home Points:  

  • Cannabis is the most commonly detected illicit drug, and THC concentrations of ≥5ng/mL increase the risk of accident culpability to OR >6. 
  • Levels of THC in the blood are higher when consumed with alcohol than when cannabis consumed alone. 
  • Visual scanning distribution is reduced to a “tunneling effect” in individuals with methamphetamine and alcohol in their system. 


Learn more

  1. The Impact of Drug Driving and the Effect of Roadside Testing By Dr. Luke Downey