Why Biological Tests of Driving Impairment are Superior to Behavioral Performance Tests

Andrew Sullivan has had a series of posts recently about “driving while stoned” (Examples here, here and here). He and a number of the people he quotes argue that driving impairment from drug consumption does not correlate perfectly with biological measures (e.g., blood alcohol level as measured by breath test, concentration of cannabis and its metabolites in drawn blood or urine), and that such tests are therefore inferior to assessing impairment directly.

Sometimes in the early morning, when I drive to work on a narrow two-lane road, I see headlights coming toward me. At those moments I hope that if the oncoming driver is alcoholic that he’s had a few belts before he got into the car, because badly shaking, sweaty hands probably impair an alcoholic driver more than does somewhat elevated blood alcohol concentration (BAC). That’s an example of the broader reality to which Sullivan points: More drug consumption does not always mean worse driving. Why then don’t we toss biological tests aside and instead rely solely on behavioral tests of driving impairment, for example reaction time tests done on a laptop, proprioception and coordination-based physical tests (e.g., touching one’s nose with eyes closed), verbal demand tests etc?

The case for such a change at first seems strong, until one recognizes that an ideal driving impairment assessment method would not only perfectly measure impairment, but would also be reliable across testers and testees. Behavioral impairment measures can’t hold a candle to biological measures in this regard.

If like me you have no life and have therefore watched many videotaped driving impairment tests done by police officers, you will have observed how hard it is to administer them in a consistent fashion. Consider the old “walk in a straight line, heel-to-toe” behavioral test. When this test is administered to a driver, both the driver and the police office are generally on the side of the road. Sometimes the ground is flat and sometimes it’s not. Sometimes the road is wet or icy and sometimes it’s dry and clear. Sometimes a strong wind is blowing and sometimes the air is still. Sometimes there is traffic zooming by which distracts both the tester and testee and sometimes the road is quiet. Those are only a few of the variables (none of which influences the result of a blood or breath test) that make it hard to administer the test reliably.

Those variables all introduce measurement error even when the best of capacities and intentions are present…and they aren’t always present. If the officer doesn’t like you for some reason, or is exhausted after a long day, or is all hyped up from the violent crime scene s/he just left, or had an argument with the spouse that morning, the behavioral performance test will be even less reliable in determining the officer’s final judgment of whether the driver is impaired. In contrast, a breathalyzer in every one of this situations will give virtually the same answer.

The reliability of biological tests is a facilitator of equal treatment under the law. Fans of behavioral impairment tests should consider whether, for example, a young black male who has had three beers and has to do a series of performance tests under the watchful eye of white cop is likely to do as well as a white, middle aged guy who has had the same amount to drink. The former driver is more likely than the latter to have his performance on behavioral impairment tests decremented by nervousness or fear of the police officer. Likewise, the immigrant who doesn’t speak English well enough to precisely understand the officer’s instructions will also be at a disadvantage relative to the native speaker. To again draw a clear contrast, if you give a blood or breath test all those factors are eliminated from the measure of impairment.

Which leaves me agreeing that perfectly reliable behavioral impairment tests would be better than what we have now, in the same sense that I agree that universal, high-quality health care at low cost would be better than what we have now. But what of it? Winston Churchill (who was probably a better driver at .08 BAC than at .00) said that “democracy is the worst form of government except all the others that have been tried”. The same could be said of biological assays of driving impairment.

[Cross-posted at The Reality-based Community]

Keith Humphreys

Keith Humphreys is a professor of psychiatry at Stanford University. He served as a senior policy advisor at the White House Office of National Drug Control Policy from 2009 to 2010.