Detection is not prevention: will drug testing drivers really make NZ roads safer?

The government’s new mandate to carry out random oral-fluid roadside drug testing marks a milestone in New Zealand’s road safety policy.

Under recently passed laws, police can now stop any driver, at any time, to screen with an oral swab for four illicit substances: THC (cannabis), cocaine, methamphetamine and MDMA (ecstasy).

Police will begin the rollout in Wellington in December, with nationwide coverage expected by mid next year.

Drivers will face an initial roadside swab taking a few minutes; a positive result triggers a second test. If confirmed, the driver will face an immediate 12-hour driving ban and have their initial sample sent to a lab for evidential testing.

With nearly a third of all road deaths involving an impairing drug, moves like this are clearly aimed at a serious problem.

Efforts by the previous Labour-led government stalled because no commercially available oral-fluid device met the evidentiary standards required at the roadside.

The government now appears to have what it needs to begin roadside testing. But it remains unclear whether this policy will achieve its goal of preventing truly impaired driving.

The science behind cannabis and driving

The research on cannabis and driving impairment is mixed. Many studies show an associative rather than causal link: people who use cannabis more often tend to report more crashes, but not whether those crashes happened while they were impaired.

Unlike alcohol – where blood-alcohol concentration closely tracks impairment – no such relationship exists for THC. Cannabis is fat-soluble, so traces linger in the body and appear in saliva long after any intoxicating effect has passed, making saliva testing a relatively poor proxy for impairment.

For the other targeted drugs – the stimulants methamphetamine, cocaine and MDMA – the connection to driving impairment is also unclear. At lower doses, stimulants can even improve certain motor skills. The risks are instead tied to perceptual shifts or lapses in attention, which a saliva test cannot detect.

Because cocaine and meth remain illegal globally, it is difficult to conduct the controlled studies needed to link presence and impairment.

The policy’s focus on just four illicit drugs also raises questions of scope. In practice, these are among the easiest and most visible substances to target: the low-hanging fruit.

Yet impairment from prescription medications such as sedatives or painkillers is far more common and remains largely self-policed.

Responsibility falls to individuals and their doctors to decide when it is safe to drive – a much bigger problem than many realise.

Police expect to conduct about 50,000 tests a year – around 136 a day nationwide – compared with more than four million alcohol breath tests annually.

While that’s a modest number, the introduction of roadside breath testing in the 1980s proved transformative. Alcohol consumption, which had been rising for decades, peaked around 1980 and then began to fall after the combined impact of breath testing and public awareness campaigns.

Whether the new drug-testing programme can produce a similar deterrent effect – without that level of visibility or education – remains to be seen.

Even if it does, the overall impact may be small. Drug use and drug-driving are far less common than alcohol use ever was, so the scope for large behavioural change is limited.

The problem of lingering traces

Another pressing question is what happens when the test detects traces of cannabis long after impairment has passed. THC can remain detectable in regular users for up to 72 hours, even though its intoxicating effects last only a few.

That means a medicinal cannabis patient who took a prescribed dose the night before – or a habitual user with high baseline levels – could therefore test positive while driving safely.

Although the law provides for a medical defence, there is still no clear procedure for proving a prescription at the roadside. Few people carry that documentation, and it’s uncertain whether digital GP records would be accepted.

In practice, some law-abiding drivers will inevitably be caught up in the process simply because of residual traces that pose no safety risk. Conversely, an inexperienced cannabis user may feel heavily impaired yet return a low reading.

This uncertainty reflects a deeper flaw in the system. When the previous government first designed the policy, it intended to test for impairment.

Because no devices could meet the evidentiary standard, the law was amended to test only for presence.

Perhaps the resulting regime’s relatively low-level penalties – such as a $200 fine and 50 demerit points for the confirmation of one “qualifying” substance – will help it withstand legal scrutiny, but they also highlight its scientific limitations.

Other jurisdictions have taken a different path. Many have returned to behavioural assessments of impairment – the traditional field-sobriety approach of observing coordination, balance and attention.

In the United States, for instance, officers often rely on such behavioural indicators because the law there still centres on proving a driver was impaired, not simply that they had used a substance.

In the end, a test that measures presence rather than impairment risks confusing detection with prevention – and may do little to make New Zealand’s roads any safer.

Joseph Boden receives funding from the New Zealand Ministry of Business, Innovation and Employment, and the Health Research Council of New Zealand. He is affiliated with the Harm Reduction Coalition of Aotearoa.