Roadside drug testing: Not even once

The Land Transport (Drug Driving) Amendment legislation was passed this month. This means that, among other things, police will be able to pull drivers over and test their saliva for THC, opiates, amphetamines, and other drugs in the same manner that they test for alcohol.

Apparently this measure will reduce the number of people driving while under the influence of illegal substances, and make our roads safer.

“Our Government is committed to reducing the serious harm that comes from driving under the influence of drugs. In both 2020 and 2019, over 100 people were killed in crashes where a driver was found to have had drugs in their system. This legislation directly addresses these preventable crashes, and will lead to safer roads for all,” Michael Wood said.

Read the rest of the official release on Parliament’s website

Read about the planned drug driving roadside tests on the Ministry of Transport website

Read the Bill on the website

Now, driving while under the influence of illegal substances can be very dangerous. If you’re stoned, your reaction times might be slow. If you’ve had speed or methamphetamine you might be tempted to take more risks, like going well over the speed limit or overtaking with no space. If you’ve had LSD you’ll be super easy to distract, so you may not even make it out of the house. If you’ve had ketamine, good luck making your legs work in the way they’re meant to in order to get to the car. Driving on salvia? Watch this to see why it’s a terrible idea.

Driving under the influence is dangerous and can be downright fatal. This is absolutely true.

Read more about the effects of different drugs on your driving on the National Institute of Drug Abuse website


Much like workplace drug testing, roadside drug testing isn’t going to make any kind of positive difference in terms of reducing these harms.

Roadside drug testing is problematic because…

Basically what’s happened is the Government looked at the programme overseas, ignored the fact that it’s a completely flawed idea that has no evidence of being effective, and ran with it anyway. Because why the heck not?

Here’s why the heck not:

Correlation is not causation. It’s bad science, and they should feel bad.

Photo of a small cat sitting in the bottom of a giant bend in a roof
This tiny kitty is obviously the cause of the busted roof. You can tell because it’s sitting on the bent bit looking super smug.

The correlation is not causation fallacy, or spurious correlation fallacy, is what happens when someone thinks that something has caused something else that is completely unrelated to happen because it

  • happened at the same time, or
  • happens in similar circumstances, or
  • happens in similar numbers.
Graph showing a false correlation between how much cheese is eaten by people and the number of people that die accidentally tangled in their bedsheets
You like cheese? You gon’ die. In your sheets. RIP. Graph source:

For example the amount of people that eat cheese is the same amount of people that wind up strangled to death by their bedsheets. The numbers are the same, so they must be the same people dying. CHEESE CAUSES STRANGULATION! CHEESE, THE SILENT KILLER!

The correlation fallacy is found in poorly thought out arguments, like movies and video games causing violent behaviour in kids, and conspiracy theories like the COVID-19 pandemic hiding 5G rollout.

Read more about spurious correlation and conspiracy theories on the European Journal of Psychology

According to the Minister’s release, drugs were found in the systems of the drivers responsible for some deaths between 2018 and 2020. According to the Minister’s statement, the drugs absolutely caused the crashes that killed people. No question about it.

This is a problem because drugs can stay in your body long past the time when you feel sober again. They might be present, but that doesn’t mean that you’ll behave erratically or drive badly and kill someone.

You could take Friday off, have a thoroughly hedonistic Friday and Saturday, recover on Sunday, and fail your test on Monday, despite not being impaired at all. You might be stone cold sober, but you still have an illegal substance in your body.

How long can substances be detected in my saliva?

The number of deaths is terrible, but they’re not hard evidence of drug-induced impairment

According to the Ministry of Transport website, 21% of the fatal crashes between 2018 and 2020 were due to people driving under the influence of illegal drugs alone, 13% were killed in crashes where alcohol alone was a contributing factor, and 11% were due to people driving under the influence of drugs and alcohol combined.

Pie chart showing the percentages of road fatalities due to drugs and/or alcohol
Graph showing the presence of drugs and/or alcohol in the bloodstream of drivers responsible for fatal crashes. Source: Ministry of Transport

It’s a pretty gnarly statistic, taking it on face value like that. So we did the maths to see if drug-driving is as much of a problem as it seems.

According to the statistics on the Ministry of Transport site, 1,048 people died on the road in 920 fatal crashes between 2018 and 2020. Of those 1,048 people, 454 people were killed by drivers with drugs and/or alcohol in their bloodstream.

Fatalities from car crashes involving people with alcohol, drugs, or both in their bloodstream 2018-220

Year Alcohol alone Drugs alone Alcohol + drugs Total deaths per year
2018 49 67 35 151
2019 46 74 39 159
2020 42 67 35 144
Total deaths per substance 137 208 109 454

These numbers are horrendous, it’s true. Over 450 people were needlessly killed in a three year period. However it comes back to the spurious correlation problem of drugs being present in the body potentially days after a driver’s taken them and is no longer impaired by them.

There’s absolutely no hard evidence that drugs are responsible for the driving that caused the fatality. The drugs were there, yes, but they could have been taken 3 days before the fatal crash happened. We can’t just categorically say that the drugs were the cause of the fatality. Yet this is exactly what the Minister is assuming when he says this law will “directly” make the roads safer.

We still need the compulsory impairment test

Something that we find particularly concerning is the fact that with the amendment to the Bill, cops won’t have to do the compulsory impairment test any more.

At the moment, if a cop thinks you’re under the influence they can pull you over and make you walk in a straight line, or count to 30 while standing on one leg or touch your nose. The tests are based on physical coordination that need you to be sober to do without falling over or messing your words up.

If you’re wasted drunk, you’ll fail the compulsory impairment test because your physical coordination is screwed. If you’ve taken drugs the cops can check your pupil size or your reaction time to different stimuli. From there they can either give you a spit test on the roadside, or bring you in to the station for a blood test.

We need this test to show whether a driver is or is not impaired, because the spit tests are unreliable. The compulsory impairment test will be removed when the amendments go through. This is a terrible idea. To quote the Medical Officer of Health in The Regional Public Health submission on the Bill;

Not allowing a CIT to be undertaken after a positive oral fluid test (for a single substance) appears unreasonable given there is poor data on correlation between detecting a substance in oral fluid and individual impairment.

Read the rest of Regional Public Health’s submission on the Bill on their website
If doctors are saying it’s a crock, then who are we to argue?

Prescription medications throw false positives for other drugs

The trouble with illegal substances is that they’re in the same families as prescription medications that people need to function on a daily basis. This means that if the cops spit test or blood test you for something illegal and you’re taking a prescription for something in that family, you’ll test positive for the illegal substance.

The tests aren’t smart enough to differentiate between, say, heroin and codeine, they just see an opioid, which isn’t great if you’re managing pain. If you manage your ADHD with ritalin, you could test positive for methamphetamine or amphetamine because it’s a stimulant.

If you’re on medication for the following things, you’re going to want to start carrying your prescription on you as evidence, just in case.

  • Depression
  • Anxiety
  • ADHD
  • Hypertension
  • Pain
  • Sleep disorders
  • Weight loss
  • Nasal congestion (severe colds)
  • Coughing
  • Hayfever
  • Acid reflux/GERD
  • Arthritis
  • UTIs and other inflammatory-type infections.

Read the full list of medications that give false positives in drug screens on the Good Rx website

It’s going to make existing inequity a whole lot worse.

We’re not saying that the police have particular targets in terms of “random” searches…

Actually nah, we’re saying exactly that. Because they do.

A fair chunk of the social problems that happen as a side-effect of poverty are associated with drugs and alcohol. So we’ve already got unfair targeting of vulnerable communities that are mostly not white.

The first infringement penalty is a $200 fine if someone fails their “random” roadside drug test. If you put that against someone on minimum wage’s earnings, or if they’re on a Jobseeker’s Benefit or Student Allowance, there’s no way they’re going to be able to pay that off. That’s going to come out of their food money or their money for bills.

Wealthy people, by contrast, will treat the fine as a minor cost, which will be unlikely to deter them. They can also afford legal aid, so if they want to dispute the charges, they actually can. Vulnerable people have to suck it up and have that against their legal record, whether they were impaired and driving dangerously or not.

Then there’s the racism.

According to the Identifying and Responding to Bias in the Criminal Justice System: A Review of International and New Zealand Research report by the Ministry of Justice, the apprehension rates in Aotearoa are super super racist:

Levels of disproportionality also differ by offence, with Māori just over three times more likely to be apprehended for drug-related offences, while almost seven times more likely to be apprehended for offences against justice, 4 and almost six times more likely to be apprehended for violent offences compared to New Zealand Europeans…

…Research shows that Pacific people are also over-represented in New Zealand’s criminal justice system, although not to the same degree as Māori. For example, Pacific people are twice as likely to be apprehended, prosecuted, and convicted, and almost 2 ½ times more likely to receive a custodial sentence or be remanded in custody than New Zealand Europeans.

That was written in 2009, and things have absolutely not gotten better in the last 13 years. When the Amendments to the Bill come through, the cops are going to have another reason to charge people with being brown behind the wheel.

Read the entire Identifying and Responding to Bias in the Criminal Justice System report on the Ministry of Justice website

Read the Over-Representation of Māori in the Criminal Justice System report on the Department of Corrections website

Also, the testing machines don’t meet accuracy standards

The machines that police here and abroad are using for this are like the Dräger DrugTest 5000. It quickly collects and analyses the person’s saliva without the person needing to spit into anything or rely on hinky reagents like you get in the “random” workplace drug tests. No mess, no fuss, done and dusted in under 10 minutes. Easy.

Except no. A piece of research done in 2019 revealed that the machines had varying sensitivity, so your spit could throw a false negative or a false positive, depending on which individual machine you were drooling on.

They also have real problems differentiating drugs if you’ve had more than one drug, so tests would be inconclusive, especially if one of those drugs was THC. Y’know, only the most popular illegal substance in Aotearoa and the one most likely to be found in people’s spit in a “random” roadside drug test.

To quote the Regional Health submission again;

The DRUID Study (H. Schulze, 2012), cited in the discussion document in point 18, the ROSITA (A.Verstraete M. P., 2001) and ROSITA-2 (A. Verstraete E. R., 2006) studies reviewed a number of interventions for drug driving and made some key recommendations for device accuracy for drug screening.

The projects recommended that devices should have a minimum sensitivity, specificity, and accuracy of 80% or higher for all drugs tested, with recommendations of 90-95% accuracy for some specific drugs. A 2019 systematic review of devices (S.C.D. Dobri, 2019), showed that none of the devices available meet these recommendations.

So, if your spit test shows a positive result for having illegal substances in your body there’s actually a high chance that it’s going to be a false positive because the machines are running at below 80% accuracy.

When you consider the impact that a positive result will have on someone’s mental health and/or employment and/or finances, that 20% is an unacceptable margin of error.

Read the DRUID study on the EMCDDA website
Read the ROSITA conference paper on the ICADTS website
Read the ROSITA-2 report on the Ruma website

Read Are oral fluid testing devices effective for the roadside detection of recent cannabis use? A systematic review on the National Library of Medicine website

There’s been roadside drug testing in Victoria since 2004, does it work?

It depends on how you’re measuring success.

Are fewer people dying? Not really…

According to the Transport Accident Commission’s (TAC) website, 41% of traffic fatalities in the last 5 years included people with drugs in their system, with stimulants and THC being the most prevalent.

There were 213 road fatalities in Victoria in 2018. Of the 213, 29.3% of them had either stimulants or THC in their bloodstream. Unfortunately the TAC website doesn’t have the stats of drivers with drugs in their bloodstream for other years, so we can only really take that year as a data point.

But in saying that, the fatality numbers for the years between 2018 and 2021 are all over the show:
2018: 213
2019: 266
2020: 211
2021: 232

It’s fair to say that, if we’re using a reduction in road fatalities as a measure of success, it’s inconclusive at best.

Read the Aussie stats on the TAC website

Are more people getting busted with drugs in their system? Yes.

The 2020 Monash University evaluation of roadside drug testing in Victoria showed an increase in detection of THC and methamphetamine in drivers that were pulled over. The program started in 2013 and is still running. So if you’re measuring success by the amount of drivers with illegal substances in their blood being caught, then yes. It’s working.

Read the full evaluation on the Monash University website

So if the tests are unreliable, don’t measure impairment at all, don’t reduce deaths on the road, don’t reduce the number of people driving with drugs in their system, and disproportionately affect people who are already marginalised, you have to ask yourself – is public safety really at the heart of this legislation? We don’t think so.

Further reading

More submissions against the Amendments:
NZ Drug Foundation
Royal New Zealand College of General Practitioners
New Zealand Medical Association
The Law Society

6 thoughts on “Roadside drug testing: Not even once

  1. The cause of road deaths is attributed by the NZ Ministry of Transport Te Manatū Waka to “drugs” if there is *any* presence of drugs found in the blood, but are attributed to alcohol only if the alcohol detected is above the legal drink driving limit. So they are not comparing like for like. This erroneous impression of drugs causing more deaths than alcohol was quoted by politicians as they passed the law, and is written into the Act itself. The correct comparison would be drugs found above the new Act’s criminal limits (although these are not, of course, based on actual impairment). This would be a much lower number.

  2. This is a terrible breach of privacy. I don’t want to talk to cops about my medication and prescriptions. They don’t have a right now know. Isn’t this discrimination against disabled people?

  3. Um, “afford Legal Aid”? Legal Aid doesn’t cost anything, that’s the point of it. It’s for people who can’t afford a lawyer. Now some who earn enough, will have to pay it back.
    However, there is a gap, people who earn too much to be eligible for Legal Aid may also still not be earning enough to afford a lawyer themselves. That is where there is an inequity issue.

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