Drug driving tests
Stuff is reporting on drugged drivers caught since the new laws were introduced in November 2009. The results show the police are a lot better than I expected at picking people to test: of 514 who had a compulsory field impairment test, 455 failed, and 429 of those tested positive for one or more illegal drugs. The drug-driving policies, at least so far, are targeting people who are a real risk (in sharp contrast to workplace drug testing, for example).
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 »
Does failure of NZ’s impairment test reliably indicate impairment? I can find info online about what the test involves, but not what constitutes a pass or failure.
The reason I ask is that, from what I’ve read in the UK context, pupil dilation/eye movement tests, while sensitive, have quite low specificity in indicating impairment. (That is, they might indicate impairment, or just fatigue, or acute stress, or hayfever, or the unusual but stable qualities of some individuals’ eyes.)
So if that element of NZ’s test alone could result in a “fail”, then the test wouldn’t be a great indicator of genuine impairment. But hopefully it doesn’t.
It’s important that the impairment test is specific, because a regular user of marijuana will test positive for it up to a week after last using it (and a one-off user for up to a couple of days), so a pupil-dilation “fail” followed by a positive blood test for marijuana can’t confirm that the driver was impaired. They might have smoked a joint three days ago, had hayever, and been fine to drive.
13 years ago
Amy,
The impairment test seems to be based on real impairment of motor control. Also, the fact that 95% of people testing positive for impairment go on to test positive for drugs is fairly convincing that it isn’t just picking up traces from prior use — the proportion of positive tests in blood samples taken at crashes is much lower, and the proportion in random non-impaired drivers should be even lower than that.
13 years ago