Are drug drivers really killing us by the droves?

For the first time drug driving deaths have eclipsed drink drivers. Or have they? Don Rowe looks at the media furore around an AA press release some experts argue is unsubstantiated. 

Last week news broke that for the first time in New Zealand drug drivers were responsible for more deaths than drink drivers. Stuff, the Herald, Newstalk ZB, even the bloody old Guardian got on board. It’s a striking headline. But is it true?

The New Zealand Herald, for example, declared: “Automobile Association study finds drugs cause more fatal crashes than alcohol.” A line in a Stuff story called it a “study by the Automobile Association.”

But the AA never undertook a study, instead relying on a single table from the NZTA’s Crash Analysis System and issuing a press release. And the data just doesn’t support the conclusions implied in the subsequent news coverage – regardless of the fact that drug driving is a obviously a very real and serious problem. 

The NZTA’s Crash Analysis System is set up to record anything that might be a contributing cause in a crash: bald tires, bad weather, an unrestrained dog frightened by a passing truck. Thomas Lumley, Professor of Biostatistics at the University of Auckland says police tend to err on the side of caution as if a factor is not entered into the database, there’s no way to go back and check it later. An analysis of CAS data in 2015 undertaken by Lumley found there were on average about 2.4 causes per crash.  

“The AA report was looking at the number of times that drugs appear in the Crash Analysis System Reports,” Lumley said. “But for something to appear on there, the police only need to have decided that it’s possibly a contributing cause. If they test and find drugs, they’re going to be relatively unlikely to conclude that it didn’t contribute.”

NZTA spokesperson Andy Knacksted agreed that in almost every crash there is more than one contributing factor, something NZTA makes clear when supplying data.

“In any crash there are typically several contributing factors. We provide a caveat to anyone using CAS that that is the case,” he said. Some drivers in the figures released by NZTA would have tested positive for both drugs and alcohol. 

And of course the presence of drugs in a driver is not the same thing as drug impairment. Heavy cannabis users may show high levels of metabolites weeks or even months after use, long after their buzz is gone and their brain is no longer thick molasses. 

The “huge swing” in statistics – up 65 deaths in five years – is likely attributable to an increase in testing, something contained in the AA report but missing in the Stuff and Herald coverage. But without a measurable and accurate threshold for impairment, the numbers don’t tell the whole story. 

AA road safety spokesman Dylan Thomsen said the AA was comfortable with their presentation of the data provided by NZTA, and that the numbers provided a fair and accurate reflection of drug-impaired drivers.

“I find it interesting as well that people don’t tend to make the same arguments about crashes involving alcohol,” he said. “When figures come out around that I haven’t seen that many people come out and say, ‘Oh, well can you prove that the alcohol was actually the cause of that crash?’”

But Professor Lumley said that while there is argument around where exactly the alcohol limit should be, the consensus is that blood alcohol is an effective method to measure intoxication. There is no equivalent solution for drug use.

“For some of these drugs there just isn’t a good test for impairment that you can do biochemically,” said Lumley. “Heavy cannabis users may not be impaired despite it being in their body. The point of testing drivers is to find impaired drivers, not to find people who smoke cannabis.”

The key driver of the AA report was to lobby government to introduce saliva testing for drivers, something they say 94 percent of their members support. But New Zealand Drug Foundation director Ross Bell said the latest research out of Canada shows saliva testing is not yet accurate enough for policing, notably missing methamphetamine and prescription medications, and providing concerning levels of false negatives for other substances.  

“One in four prescriptions in New Zealand are for medications that impair your ability to drive and these are the things that saliva testing can’t see,” said Bell. “Saliva testing is expensive and arduous and the research shows it’s just not there yet.”

Thomsen said the AA was aware of concerns around accuracy, but maintained it was better than no testing at all.

“They don’t catch every single person that has drugs in their system but our view is that it’s much better to be detecting 90 percent of people than to be detecting none,” he said. “We think that drugs are a huge factor in terms of fatal and injury crashes on our roads, so this is something that we are going to continue to be pushing for.”

Lumley urged caution. “I think it’s a really bad idea to spend lots of money on tests that don’t detect the thing you care about. Look at meth contaminated houses, what we care about is houses that are at dangerous levels but that’s not what we tested for.”

A members bill by National MP Jami-Lee Ross which would allow for roadside testing for cannabis, ecstasy and methamphetamine is currently before parliament, and Associate Transport Minister Julie Anne Genter will be under pressure to respond to last week’s wave of media coverage.

But while drug driving is a serious problem and one worth addressing, the meth testing crisis demonstrated it is imperative we scrutinise press releases much closer before committing large amounts of public money to inefficient programmes.  

“Press releases often get adopted uncritically by journalists, particularly in situations where there’s a potential for moral scare,” said Lumley. “But it’s important to think critically about risk and also about what real solutions may be.”

“It makes a better headline, it could be true, but there’s not much support for it in that data, and there’s no straightforward way to say if those crashes were caused by drugs.”


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