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

Biological tests of driving impairment are lousy until you compare them to every other kind of test

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.

Author: Keith Humphreys

Keith Humphreys is the Esther Ting Memorial Professor of Psychiatry at Stanford University and an Honorary Professor of Psychiatry at Kings College London. His research, teaching and writing have focused on addictive disorders, self-help organizations (e.g., breast cancer support groups, Alcoholics Anonymous), evaluation research methods, and public policy related to health care, mental illness, veterans, drugs, crime and correctional systems. Professor Humphreys' over 300 scholarly articles, monographs and books have been cited over thirteen thousand times by scientific colleagues. He is a regular contributor to Washington Post and has also written for the New York Times, Wall Street Journal, Washington Monthly, San Francisco Chronicle, The Guardian (UK), The Telegraph (UK), Times Higher Education (UK), Crossbow (UK) and other media outlets.

28 thoughts on “Why Biological Tests of Driving Impairment are Superior to Behavioral Performance Tests”

  1. Another consideration might be contextual: why is the test being administered at all? why was the driver pulled over?

    If the driver has been pulled over, because of some reckless maneuver or traffic violation, then the officer has already observed behavior and found it wanting. The biological measure has the advantage that it is corroboration. A further observation of behavior is redundant, and the officer may abuse his authority, consciously or unconsciously, without fear of contradiction. The breath test or blood test stands as a useful control on various types of error.

    On the other hand, I think we might feel somewhat differently, about, say, a road block, where drivers are being tested at random, without any violation of law, or even suspicion. Would we consider a concentration of cannabis and its metabolites in drawn blood or urine to be sufficient evidence of a crime, with no other specific circumstance? This would be, stop and frisk at a whole new level.

  2. The standard behavioral tests for impairment are also great tests for autism. Follow the light with your eyes without turning your head? Yeah, well I’m slightly cross-eyed, very sensitive to bright lights and, like a lot of autistic people, I instinctively turn my head rather than follow with my eyes. Walk a straight line heel-to-toe? Are you kidding me? My sense of balance is hopeless for that kind of thing.

    I do a lot of late night driving. (Another autism symptom is strange sleep cycles.) I’ve been pulled over several times for things like having a burned out headlight. If they decide to get me out of the car and apply their test I ask them if we can just skip straight to the breathalyzer because I can’t pass the behavioral tests under any conditions.

    1. I have a similar concern since I wear hearing aids. I can kind of get by without them but even with them comprehension is not always good. We annually travel to Canada. I make sure my wife is driving when we cross the border because if the Canadian officer is a native French speaker I have a very difficult time with comprehension. That makes me nervous and could possibly flag a search for no reason.

      If I am pulled over I am similarly nervous and my lack of comprehension could be misinterpreted.

  3. All of this is true as long as the biological result is a reasonable proxy for the intoxicating effect of the drug, as BAC is for alcohol. If that’s not true for cannabis, then the fact that the test is “fair” might not matter much; flipping a coin would also be “fair.” The harder problem is cannabis (and most of the pharmaceuticals) have no non-invasive test equivalent to the breathalyzer. You can’t do random blood tests, which means the inquiry needs to start with some evidence of functional impairment.

    1. Mark wrote If that’s not true for cannabis, then the fact that the test is “fair” might not matter much

      You are mixing terms in a way that I don’t think will help comprehension. I said the tests are reliable. Fair is a judgement about both reliability and validity.

      flipping a coin would also be “fair.”

      This is what I mean about mixing terms not being helpful. A coin flip would not be fair (both reliable and valid). Flipping a coin is not reliable, if you test the same person twice you have a 50% chance of getting a different answer, whereas with biologicals you have a less than 1% chance of that. As you know, without reliability, there can be no validity (and no fairness either).

      1. True enough. But if the test isn’t valid, it doesn’t matter how reliable it is. So the question is how close the correlation is between blood THC concentration and actual impairment. Given that DUI is a serious crime, even a modest risk of convicting someone who hasn’t actually done anything wrong is to be avoided. There’s no evidence that I know of that someone who got stoned Friday night is a worse-than-normal driver Monday morning. So a rule that makes such a person subject to a DUI conviction strikes me as a bad rule, even if the test-to-test reliability is .99.

        1. I think Mark nailed this argument. The biological tests are derived from extensive statistical sampling, which of course leads to unfair treatment of individuals, but it seems that all such laws are like that. They are compromises deemed to be in the best public interest, but the best public interest is not necessarily fair, I who can easily consume four beers and continue to drive safely am put at a disadvantage by the biological test, but I would rather I have this set of laws than none at all.

        2. No criminalist that I know of will testify that a specific level of delta-9 THC is impairment per se (although they will testify that a high ration of delta-9 to carboxy indicates recent use). The reason for this is that there is simply a dearth of scientifically meaningful impairment correlation studies – a spillover from marijuana’s classification as a Schedule 1 drug. This may change if and when marijuana is legalized.

          Until then, the criminalists want to see things like bad driving and performance on standardized field sobriety tests. They also want to see the results of a drug recognition evaluation, in which specially trained officers check suspects for clinicals and objective signs of drug use, and perform another battery of field sobriety tests. (And in my experience, these officers are pretty good, even in polydrug cases that give confusing and sometimes conflicting data.)

        3. @Mark — yes if a test isn’t valid, it doesn’t matter how reliable it is. It does not follow from this that you can reduce reliability and gain validity. You can’t.

          I don’t think your view is that there should be no limit on how much cannabis a driver can have on board, but that the current limit is too low. That isn’t a case against biological tests. If you increased the standard dramatically you will still want a reliable test.

          But perhaps I am misunderstanding: Presuming you think there should be some standard, what is your reliable and valid way to test if someone is over it?

  4. As a young white man, I failed a field sobriety test stone cold sober. I’m more-than-average clumsy, the kid picked next-to-last in grade school sports. The breath-o-meter saved me a lot of lawyer fees; I got a zero. Still spent most the night in jail.

  5. Did Winston Churchill have a driving licence? He spent most of his mature life in London, where out of office you hop into a taxi, top hat and all; and in office, are chauffeured around. He spent much of his adventurous youth in parts of the world without any roads. Rudyard Kipling, a rich technology buff who owned several cars, didn’t drive any of them himself.

  6. Given the facts above (which I don’t disagree with, except for the caveat noted by Mark Kleiman), wouldn’t the obvious solution be that you need to fail BOTH a biological and a behavioural test to be found to be presumptively impaired?

    1. That depends on where you live and what you have been taking.

      In California, if you have been drinking, you are “presumptively” impaired if you are at or above .08 percent within three hours of driving, whether or not the officer administers field sobriety tests. (Vehicle Code section 23152(a).) You are also “presumptively” guilty of driving with a BAC at or above .08 percent, which is a distinct criminal charge. (Vehicle Code section 23152(b).)

      However, for drugs, there are not enough impairment correlation studies for the scientific community to reach any sort of consensus about presumptive impairment. In these cases, the prosecutor must show that you would have been impaired, and the tox result is merely one piece of circumstantial evidence showing impairment.

  7. Behavioral tests are also subject to Type II errors. There are plenty of people who, when moderately impaired, can pull it together to focus on a very important task. But the minute the rush goes down and the attention wanders…

    Meanwhile, the question about marijuana just highlights how stupid and counterproductive the drug warriors’ demonization of it has been. If the drug warriors gave a s*** about public health, there would be reams of high-quality results on the effects of marijuana on reaction time, overall cognition, executive function and so forth. There would even be a bunch of competing technologies for determining body levels of cannabinoids. But instead, it’s like abstinence-only birth control.

  8. After decades of trial and error, many states have made high BAC level an independent offense, without proof of impairment, and I have little trouble with that. One is in near-total control of one’s own BAC level, and there is little doubt that as the level edges up self-awareness of impairment level becomes every more chancy. I think it is reasonable for society to say, “If you ingest significant alcohol, it is too dangerous to let you drive, regardless of your ability to talk clearly or walk a straight line.” (How you get to the point of a blood test is a related but separate issue.) I understand that some European countries do that at much lower levels than the U.S.-standard .08, and even here it is often .00 for under 21’s. As Mark says, this does not transfer well to THC.

  9. What I think I’d favor is a zero BAC limit for anyone with THC on board. That’s still not perfect, but it’s better, and completely controllable. You just have to either not drink or not drive after drinking for some days after toking up.

    1. Does the residual THC become psychoactive or somehow potentiate the alcohol? If not, won’t zero BAC w/THC ensnare many unimpaired drivers?

      1. Yes, some people who choose voluntarily to get stoned and then choose again voluntarily to drink and choose once more voluntarily not to wait until their BAC is back to zero before driving will be subject to punishment. That seems to me a reasonable way of dealing with the fact that alcohol and cannabis both contribute to the risk of driving without making it a crime drive for a week after using cannabis.

    2. I see no reason for zero on THC, if you want to pick one of a number of factors that might be measured biologically connected to the eeevil weed. I have to be immobile for the weekend to keep on the legal side of my Friday night joint? Ridiculous and not going to happen.

      If society can manage with all these folks driving around at somewhere less than .08% then they need to get used to the idea that people have and are driving in some state of mellowness that is less than DUID all along, too — because if there was some dramatic need for zero THC it would have been evident by now. Even if there was a way to go all vampire on my blood without a warrant.

      People should keep in mind that the whole idea of per se laws is to offer a slam dunk to the arresting officer and prosecutor. They are not designed to prevent anyone with zero alcohol on board from driving. Is gov’t supposed to protect us from every risk, no matter how tiny? Doesn’t look that way to me, but some folks think gov’t needs more duties that seem ill-advised in nations that don’t happen to resemble North Korea in terms of their intrusiveness.

      1. Remember that 0.08 mg/dL is the presumptive level (in most jurisdictions). A driver who is demonstrably impaired below that level can be convicted of DUI. The police and the DA just have to work harder than they do when someone blows a 0.10.

        Demonstrating impairment involves precisely the sorts of biological tests used in the days before the breathalyzer became ubiquitous. I presume the tests are now recorded as evidence.

      2. You didn’t read Mark correctly, I think. You don’t have to be immobile for the weekend after a Friday night joint. You would have to be immobile for Saturday if you followed your Friday night joint with a Saturday beer. Essentially you can choose two of the three: marijuana; a beer or two; or driving. If you want to use both drugs then you can’t drive until at least one of them is completely out of your system.

        I have no idea if Mark’s idea is a good one, but I think that’s a better representation of it.

        1. J. Michael,
          Yeah, blundered writing what I meant in full while running out the door to another engagement. Badly stated as it may be, my position stands. I understand about the BAC vs DUID point Mark was making. I just object to it based on 40 years of experience. Anecdotal, it may be, but it works for me. I just don’t see this long-term, residual THC/BAC interaction that Mark is tweezering out of some statistical study.

          I guess I don’t have a personal worry in this respect, just like millions of other personally responsible marijuana users. I’m a cautious, safe driver — attested to by the local DMV — rarely consume more than two drinks at this point in my life, and smoke responsibly with what I consider to be suitable intervals before operating heavy machinery. Frankly, before teasing something else from statistics here, just do a some serious experimentation and turn a bunch of us loose on a closed track with suitable comparison and control groups. When the bong and bar are ready let me know. I wouldn’t doubt there are deficiencies from time to time, but me experience tells me that getting a good night’s sleep will make up operational deficits a lot more than worrying if I’ve waited enough days before I crack open a frosty or go for a drive afterwards (within reason, of course, I’m not arguing slamming a six-pack or anything silly.)

          Once again, if we allow worst case scenarios to guide us in what freedoms we can exercise today, well, the terrorists have already won.

          Call me a cynic about this sorta stuff if you will. My own work deals in part with risk perception, specifically about nuclear power. The gov’t wanted us all to feel safe around those happy nukes that would be spitting out power too cheap to meter. Lots of studies and statistics, only these were all shaved the other way. “Trust us! We know what we’re doing with nuclear power. We’ve hired ENGINEERS!…” Weird how this knife slices both ways with trying to recoup marginal findings, mindbogglingly incomplete problem definition, and oh so convenient findings…”There’s a menacing policy problem…but look what I just found!”

          All I’m suggesting is that there needs to be a whole lotta science going on with per se re weed. Right now, it’s all policy…and as I’ve commented several times recently, policy ain’t science, or we’d be ruled by intelligent, patient people who had the public’s best interest at heart.

          1. From your work on nuclear, you point out that ” science” speaks to both sides of the issue. Surely that is the case with whatever we know about drug use. Mark seems to be coming down on a highly restrictive policy derived from some number of scientific findings to which he is privy, but there seem to be other findings which contradict his position. I just don’t feel that any science is going to bring us to a reasonable policy. It’s not working with climate change, keystone pipeline, coal shipments to China. Science is just as subject to human error and self interest as any other human endeavor.

          2. anonymous,

            Be cautious in assessing blame to “science” when it comes to policy making. Science is about objective information constructed by subjective humans. Policy making as practiced in the United States makes reference to science, but otherwise has little formal attachment to conforming to the scientific method. Except for those entirely motivated by religion (obviously NOT science) even the climate deniers try to make their case by cherry-picking and misrepresenting facts drawn from science.

            The root of politics and policy is the same for all practical purposes. Policy is simply politics realized. That’s usually NOT science, unfortunately. Bad policy is quite often rooted in politically miscasting and misusing science. That how we ended up with marijuana prohibition in the first place and it was an endemic feature of the original Progressive movement, which is part of the reason we ended up with alcohol prohibition. Politicians know a sucker is born every minute, thus their ready deployment of “science” in various ways to charm the rubes. So far, the rubes are still buying, but at least with marijuana, we’re at the tipping point. Thus my concern about conceding this point on the basis of “science.”

  10. I don’t understand why we can’t have it all.

    Why not make passing either a behavioral test or a biological test enough to avoid a DUI charge?

    1. @bymotov: We could make the standard different. There is nothing sacred about current levels (BAC standards have varied over time and vary across states and countries). It is a question of utilities, how much we balance reduction in road deaths/injuries with the risk of unnecessary arrests.

      But we can’t have it all — policy is about choices and tradeoffs.

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