Back to top

[ Skip to main content ]

Driving high


hand with roll-up on steering wheel

Cannabis and cars don’t mix. We know pot causes impairment, but just how much, and is it even that dangerous? Damian Christie looks at one of the emerging issues around cannabis harm.

“I drive better when I’ve been drinking” is not something you hear these days. Whether it’s down to personal experience, years of watching the horrific results of drunk driving on the news or the millions spent on a variety of advertising campaigns, the message – for most of us – has sunk in. Drink, Drive, Bloody Idiot, Ghost Chips etc.

Despite the obvious logic – things that make our brain go funny don’t improve our driving – there seems to be something stopping people who regularly use cannabis from reaching the same conclusion. It’d be easy to attribute the disconnect straight back to their drug of choice, but the reality is more complex, as is how best to detect and deal with those who continue to drive under the influence. The answer to those questions is a source of debate not just in New Zealand but around the world, where even the experts disagree on some fundamentals.

Not surprisingly, in New Zealand, more people drive under the influence of cannabis than any other illegal drug. What is surprising, though, is that more people report driving while impaired by cannabis than over the limit for alcohol. The data isn’t perfect but would suggest at least one in five New Zealand drivers has driven under the influence of cannabis – within three hours of smoking – in the past year. Two-thirds of cannabis users report drug driving in the past year, and most rate it as far less dangerous than driving under the influence of alcohol.

Among younger people, the problem is even greater – a Canadian study found 40 percent of people aged 15–24 had driven stoned – double the rate who said they’d driven under the influence of alcohol. More worryingly, they’d done it not just once, but on average 10 times in the past year – far higher than the same figure for alcohol.

The rhetoric around driving stoned will be familiar to many: “It makes me a safer driver.” “I drive slower when I’m stoned.” “I’m more careful.” Is it true? In a word: no. But exactly how dangerous is it? And if we were to set some sort of limit in the same way as we do with drink driving, how stoned is too stoned to drive?

There is no doubt cannabis impairs driving ability, says Dr Barry K Logan, “within limits”. Based at Philadelphia’s NMS Labs, Dr Logan is one of the world’s foremost experts in drug-impaired driving, although he says most cannabis users’ chosen level of impairment is not particularly high.

“A user-preferred dose produces a level of impairment equivalent to a moderate level of alcohol consumption, 0.04 percent to 0.05 percent [blood alcohol concentration or BAC] for about 2–4 hours. And then after that, the evidence is people pretty much return to the baseline.”

It’s worth noting New Zealand’s blood alcohol concentration for drink driving is – rather controversially – set even higher, at 0.08 percent. The upshot is most moderate cannabis users don’t get higher than the level we already deem acceptable for alcohol.

“People don’t enjoy it,” says Dr Logan, “there’s a certain level of cannabis use in periodic users where it’s not really fun any more, it’s almost self-regulating.”

Which is not to say it’s ‘safe’ – blood taken from Canadian drivers involved in fatal accidents shows drivers who test positive for cannabis are five times more likely to die than sober drivers. This is slightly lower than those found with alcohol in their system. But – and it’s a big but – when cannabis is combined with alcohol, the risk of a fatal accident jumps to 40 times more likely than a sober driver. And that risk is present even just with moderate levels of cannabis and blood alcohol under the drink-driving limit.

For regular smokers, the news is worse. A new study shows that chronic, heavy users of cannabis are not, as one might think, less impaired due to higher tolerance but in fact may be constantly impaired – even for some weeks after ceasing altogether.

So if driving under the influence of cannabis, with or without alcohol, always presents some level of danger and sometimes a very high one, the question is, how best to police it? Current enforcement differs from country to country, state to state.

In states where cannabis possession is illegal, it’s easy to impose a zero-tolerance approach. Having cannabis in your system might not necessarily mean you’re a danger on the roads, but it does show you were up to no good.

But for some experts working in this field, mixing drug enforcement with traffic safety is not the way to go, confusing two distinct purposes and creating a law people don’t respect. And in states where cannabis is legal for medical or recreational use – which is a growing number – another approach must be found. The options include requiring proof of impairment or setting a ‘per se’ limit. Like the system for drink driving, this establishes a defined limit of THC in the blood, over which a driver is considered legally impaired, regardless of whether they can stand on one leg or not.

Despite cannabis remaining illegal in New Zealand, rather than a zero-tolerance approach, our law does require proof of impairment – once alcohol is ruled out through roadside screening, drivers suspected of being impaired by drugs are subjected to a standardised field sobriety test (SFST).

To the untrained observer, the SFST might seem something like guesswork, but when performed by a trained officer, it’s surprisingly effective. Developed in the 1970s, before the advent of alcohol breathalyser technology, the SFST was originally used to detect drunk drivers but is now routinely used to test for drugs. Amy Porath-Waller from the Canadian Centre on Substance Abuse has been studying the test and whether it’s fit for this new-found purpose. By comparing thousands of roadside SFST evaluations with the subsequent blood samples given, Porath-Waller says the answer is an unequivocal ‘yes’.

“What our results are showing is that you are able to predict, from these four different classes of drugs we studied, the officer is able to detect if they’re impaired and the type of drug responsible for that impairment.”

In general, each of the common classes of drugs leaves its own ‘fingerprint’ under the SFST. So, for instance, cannabis-impaired drivers will have more difficulty with the ‘standing on one leg’ test than the ‘walk and turn’ test, where an alcohol-impaired driver would struggle with both. Similarly, the test for nystagmus (involuntary twitching of the eye) will pick up those affected by depressants but not cannabis.

If there’s an issue with the SFST, it’s the degree of training required and the time it takes to administer each test. For those enforcing the law, it doesn’t provide the ‘anywhere, any time’ sort of disincentive that people now associate with random roadside testing for alcohol.

New Zealand’s head of road policing, Superintendent Carey Griffiths, says, while drink driving remains the priority, next year will see further attention given to drug-impaired driver testing, including the best approach to take.

“My preference based on what I’ve seen and know so far is a saliva-based testing regime similar to what’s conducted in many of the Australian jurisdictions, because there is an element of general deterrence to that.”

Saliva testing might be quicker than a full SFST test but can still take up to eight minutes to administer, meaning it’s impractical to use the driftnet approach of alcohol checkpoints. The single-use disposable test kits aren’t cheap either, and overseas experience has shown widespread random testing isn’t particularly effective.

In 2009, Victorian Police randomly tested nearly 28,000 motorists and found just 300 tested positive for drugs – around 1 percent. Canadian research also shows that, despite having a high rate of drug-impaired driving, in 2011, just 1.4 percent of total impaired convictions were for drugs. On those numbers, an average driver could drive under the influence of drugs 16,500 times – or every day for 45 years – before being charged.

In Australian states such as Victoria and Queensland, they’ve moved away from random testing towards an intelligence-based approach, pinpointing neighbourhoods and locations where drug use is more likely. Queensland’s zero-tolerance legislation only requires the presence of cannabis – there’s no need for impairment. If New Zealand were to head in this direction, we’d need to legislate to remove impairment from the offence.

The other option, says Superintendent Griffiths, is to consider a per se limit.

“It would be possible to get a panel together to determine an impairment level under which you’re at a legal zero, so you’re not dealing with residual effects. And you can set impairment levels for different drugs and their analogues, so that’s a way around it.”

Much time and effort has gone in to studying the level of impairment caused by ingesting different quantities of illegal and legal drugs and trying to establish an equivalent to the 0.05 percent BAC level. On paper, it seems a sensible approach, and it’s been adopted in Washington and Colorado, where cannabis has recently been legalised.

“A per se limit for cannabis is particularly problematic,” says Dr Logan, who points out there’s a big difference between testing in a lab and real-world enforcement. The time between someone being stopped by Police and giving a blood sample can easily be two hours. The active element of THC in cannabis metabolises, at least initially, much faster than alcohol and other drugs. This means a driver who was over the limit when they were caught may be well under by the time they give blood.

The method Dr Logan prefers – regardless of whether cannabis is illegal – is very close to what we currently have in

New Zealand: proof of impairment followed by proof of cannabis in the driver’s system, whatever the level.

“It’s really going to have to come back to good investigative Police work, and if you get some objective evidence that the person is under the influence of the drug then you base the prosecution on that. You say on the one hand the observations show the person was impaired, and you have a chemical test saying cannabis was in their system, and the court can decide if one is related to the other.”

Rather than blood tests, Dr Logan prefers an evidential oral fluid sample, as it can be collected roadside, usually within 15 minutes of driving.

One further advantage the impairment approach has over setting per se limits is the complexity around people using more than one drug at a time, known as polydrug use. As mentioned, alcohol and cannabis combine with a potentially lethal effect, even at levels where individually they would be acceptable under a per se regime. Add other drugs to the mix, in endless possible combinations, and it could become a legislative and judicial mess, whereas impairment is impairment, regardless of the cause.

This approach might not offer much in the way of a general deterrent, however, and Superintendent Griffiths says, while New Zealand’s drink-drive policy has an impact on the whole range of drinkers, the current drug policy is only dealing with people “at the top end of the curve”.

“I think until we deal with behaviours across the board – and that’s where a lot of the advertising is targeted at the moment – until we get into that space in an enforcement sense, I think we’ll just keep chipping away at the problem long term without making massive gains.”

Research currently being done by the Ministry of Transport with ESR looking at blood taken from drivers in fatal crashes should shed some light on New Zealand’s particular problem. At present, if a driver tests positive for alcohol, no further tests are required, making it difficult to get a firm grasp on the prevalence of poly-drug use. The research should give a clearer idea of the risks, although Superintendent Griffiths says, even then, we should approach with caution.

“I call it the sausages argument. You get five drivers who’ve crashed, and they’ve got sausages in the boot of the car. It doesn’t mean the sausages have caused the crash. People who consume large amounts of cannabis may also exhibit other incivilities, which can include [unsafe] driving behaviours; it might be that the type of person who crashes is the type of person who has cannabis in their system. I’m very careful not to mix correlation with causation.”

Damian Christie is an Auckland-based journalist. 

Photo credit: independent.co.uk

Damian Christie
Auckland-based journalist

Check out these related articles: See more articles

Survey reveals drug driving concern

Results from the Drug Foundation’s survey into driving and drugs suggest “cannabis driving” is a serious road safety issue in New Zealand....

Read More

Steer Clear wins health improvement award

The team behind the Steer Clear road safety campaign received a Ko Awatea 2015 International Excellence in Health Improvement Award in Auckl......

Read More

2nd International Symposium on Drugs and Driving

The 2nd International Symposium on Drugs and Driving was hosted in Wellington to confront the challenges of responding to drug impaired driv......

Read More
aug 2011 dope driving

Mythbusters: Dope driving: I'm a safer driver when I'm stoned - crap or fact?

Many pot smokers who smoke and drive will say they're fine to get behind the wheel. Mythbusters looks at the evidence to see whether this is......

Read More
Share:
e substance newsletter promotional thumbnail
Subscribe to email updates

Get regular news, analysis and commentary on drugs issues in New Zealand. Free.

Sign up now!

DF WebAd DrugHelp 4
DrugHelp

Online support resource if you or someone you care about is struggling with drug use

Visit DrugHelp