The drug-driving problem
The AA are campaigning again for random drug tests of drivers. I’m happy to stipulate that in NZ lots of people smoke cannabis, and some of these people drive when stoned, and sometimes when drunk as well, and this is bad. As the ads say.
On the other hand, science has not yet provided us with a good biochemical roadside test for impairment from cannabis. For alcohol, yes. For THC, no. That’s even more of an issue in the US states where recreational marijuana use is legal, since the option of just taking away driving licences for anyone with detectable levels isn’t even there.
This isn’t just a point about natural justice. There’s empirical reason (though not conclusive) to believe that many people who might fail a biochemical test are reasonably careful about driving while high.
First, there hasn’t been any evidence of an increase in road deaths in the US states where medical or recreational marijuana use is legal, even though there has been an increase in people driving with detectable levels of the drug.
Second, if you look at the 2010 ESR report (PDF) that the AA are relying on, you find (p20)
The culpability of the drivers using cannabis by itself was determined and odds ratios have been calculated as described in the alcohol section and in Appendix two. The results are given in Table seven. The odds ratio calculated for cannabis only use is only slightly greater than one, implying that cannabis does not significantly impact on the likelihood of having a crash.
Now, the report says, correctly, that this disagrees with other evidence and that we shouldn’t assume driving while stoned is safe. But they tried quite hard to do alternative analyses showing cannabis was bad, and were unsuccessful.
In 2012, there was another AA campaign, and a story in the Herald
But Associate Minister of Transport Simon Bridges said the Government would wait for saliva testing technology to improve before using it.
A government review of the drug testing regime in May concluded the testing devices were not reliable or fast enough to be effective.
It ruled the saliva screening takes at least five minutes, is unlikely to detect half of cannabis users, and results are not reliable enough for criminal prosecution.
“The real factor is reliability … we can’t have innocent people accused of drug driving if they haven’t been.
“But as the technology improves, I’m sure in the future we will have a randomised roadside drug test.”
That seems like a sensible policy.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
“screening takes at least five minutes, is unlikely to detect half of cannabis users”
Of course they say half of advertising is wasted , but they dont know which half.
A cannabis test which only detects half of users is almost certainly missing the ‘lower half’ or those only marginally impaired. Which in a way is the result that they are after. Same goes for speeding and most other driving offences.
8 years ago
Not in this case — in contrast to speeding, where the estimated speed is very strongly related to the true speed and the borderline cases are not as serious. Or to alcohol.
The whole problem is that the concentrations measured by the saliva test for cannabis are *not* all that highly correlated with impairment. It really is a more difficult screening problem than the ones we have a good solution to. See eg this NY Times article, (and where the US version of the AA is taking the opposite position.)
8 years ago