4 Questions That Uncover the Problem With Our Current Drug Driving Laws (& Medicinal Cannabis)

Read time: 2.8 min read

This article is written by Drive Change ambassador Michael White. Below, he shares his experienced insight and analysis of recent data on medicinal cannabis and its effects on driving.


As a person who has been involved in drug-driving policy and research for over 25 years, I have recently been asked to provide answers to the following four questions in relation to the road-safety implications of driving after using cannabis. 

These are probing questions that get to the heart of the matter. They are the questions that policy makers need answers to, if they are to develop evidence-based policies in relation to driving after the use of cannabis.

Q1. Does research show that risk of crashing when there is THC in the bloodstream are significantly higher than when there is no THC in the bloodstream (based on an unbiased odds-ratio calculation)? 

My answer is ‘no’ – when study biases are taken into account. 

In other words, the presence of THC in the blood is not associated with a higher risk (odds ratio) of crashing. To come to this conclusion I reference a recently completed epidemiological review1 of the cannabis-crash literature that I have co-authored with my colleague Prof Nick Burns (White & Burns, 2021), which is that the null-hypothesis (cannabis-crash odds ratio = 1.0) cannot be rejected. 

However, that statement of our conclusion is, of course, compatible with the true odds ratio being slightly greater than 1.0 – such as 1.2. However, my opinion is that the true odds ratio most probably really is 1.0. 

Q2. Does research show that driving performance is degraded when there is THC in the bloodstream compared to when there is no THC in the bloodstream?

Yes, so we have a problem. 

Why does the degradation of performance (impairment) not increase the cannabis-crash odds ratio? There are many things to be taken into account. Cannabis users are not inclined to drive soon after using, especially if they feel intoxicated. Cannabis impairment is generally weak, and probably normally below a threshold where driving performance is dangerously degraded. 

Furthermore, while there could be a small increase in crash risk due to impairment, it is probably balanced by a small decrease in crash risk due to more cautious driving (as demonstrated by slower travelling speeds and greater headways).2

Given the role of speed in crash causation, a little extra caution could go a long way. Nick Burns and I are working together again to review the literature of the impairing effects of cannabis.

Q3. If medicinal cannabis has effects on driving performance, then is there a time window after ingestion of THC when impairment is most likely to degrade driving performance? 

Yes, there is a time window. Its duration has been explored in a couple of recent reviews.3 4 The most conclusive evidence we have so far is this:

For smoked cannabis, it is probably about three hours from the moment it is ingested. For edibles, such as cookies, it is probably a bit longer, starting after a delay of at least half-an-hour. The two aforementioned reviews also indicate that low-level impairment can, in some cases, last beyond three or four hours.

Q4. Does a performance-based test exist which can measure any decrease in performance that might arise from having THC in the bloodstream?

No. There is too much individual variation in any performance tests and too much individual variation to be able to accurately gauge levels of THC impairment. However, the answer could possibly be ‘yes’ if baseline levels of performance could be established for individuals. 

Still, even then, the measured performance decrement due to THC would probably be irrelevant to crash risk.  

There is a smartphone app known as the ‘DRUID test’ that seems to be popular in some quarters for measuring driver impairment. A recently published paper that includes an evaluation of the test can be viewed from the journal’s website here.


  1. For a copy of this review, please email a request to mawhite8@internode.on.net.
  2. White, M. A., & Burns, N. R. (2021). The risk of being culpable for or involved in a road crash after using cannabis: A systematic review and meta-analyses. Submitted for publication.
  3. Eadie, L., Lo, L. A., Christiansen, A., Brubacher, J. R., Barr, A. M., Panenka, W. J., & MacCallum, C. A. (2021). Duration of neurocognitive impairment with medical cannabis use: A scoping review. Frontiers in Psychiatry, 12, Article 638962.
  4. McCartney, D., Arkell, T. R., Irwin, C., & McGregor, I. S. (2021). Determining the magnitude and duration of acute delta-9-tetrahydrocannabinol-induced driving and cognitive impairment: A systematic and meta-analytic review. Neuroscience and Biobehavioral Reviews, in press.

One Size Does Not Fit All: Mick Palmer on the Need for Fit-For-Purpose Driving Laws in Australia

Read time: 3 minutes

This article is written by Drive Change ambassador and former Commission of the Australian Federal Police, Mick Palmer. Below, he shares his take on where the laws need improvement to protect medicinal cannabis patients and the community on the road.


There is a clear and well-understood correlation between impaired driving and road safety. Australia’s road toll is of broad community concern, and the Government’s effective policies–like the .05 drink driving legislation and speed cameras– should be applauded. However, Australia’s ‘zero-tolerance’ drug driving policy creates trouble for both patients and police. 

“Fit-for-Purpose”

The government has already allowed for doctor-approved access to medicinal cannabis. This comes in response to the ever-growing scientific evidence, in addition to anecdotal evidence, that medicinal cannabis provides relief for people experiencing genuine pain and suffering due to a range of health issues, including terminal illness.

The government’s recognition of this evidence provides a step toward relief for thousands of Australians. However, for the legality of medicinal cannabis to fully operate as intended, the roadside drug testing protocol urgently requires amendment. 

I am not suggesting impaired driving laws should be weakened, but rather that they are updated to fit-for-purpose and aimed at achieving intended outcomes. 

To become fit-for-purpose, retain credibility, and operate fairly, driving laws must be aimed at dealing with impaired driving. Until this happens, many law-abiding Australians suffering from illness and often extreme pain will continue to experience legal and physical distress as a direct result of the discriminatory drug driving laws. 

“Police Leniency”

Likewise, police have a clear responsibility to effectively manage and apply the legislation in the community interest. The truth is that police exercise considerable compassionate discretion in their enforcement of drug driving laws every day. This is, in part, due to the driving laws being unfit-for-purpose and in need of review and amendment, or repeal.

The role of the police is to enforce laws, not make them. It is certainly a recipe for inconsistency to expect the police to cure legislative deficiencies by making personal judgments on patients. 

Furthermore, if police are screening for medicinal cannabis use but not for impairment, the laws are unfit-for-purpose and end up punishing people already coping with a terminal illness or serious pain.

Medicinal cannabis patients should not need to rely on the favourable exercise of discretion by police to escape punishment. It is up to Governments to have the courage and decency to legislate the necessary changes.

“Necessary Changes”

If there is any chance that medicinal cannabis can help people with terminal illness or serious pain, to lessen their pain and suffering and improve their quality of life, we should, as an educated and compassionate society, do all that is reasonably possible to facilitate access. 

The fact that these current drug driving laws exist is sufficient for many patients who qualify for medicinal cannabis treatment to decide not to take their medication. 

Some of these patients need to drive their motor vehicles as a matter of daily necessity. Sufferers should not be forced into situations where they may end up breaking the law to gain relief. Surely this is not the government’s intention.

The reality of current roadside testing is that they are not only discriminatory, but also invariably lead to further distress, fear, and embarrassment for the patients.

The resulting pain and suffering achieve the exact opposite of the intention of Governments in allowing legal access to medicinal cannabis in the first place. I am personally confident that this also is not the intention.

The facts speak for themselves. There is a better way to uphold law and road safety. We must be prepared to explore the options. 

As an example, Norway has successfully been assessing drug levels in driver’s systems for many years. This surely is far preferable to the continuation of a “zero-tolerance” approach that we have here, which can only be beaten by judicial discretion.

The current medicinal cannabis drug testing laws need to be subject to urgent review. By amending the laws, we move to ensure they operate fairly and without discrimination while continuing to achieve the intended outcome of controlling impaired and potentially dangerous driving.