What Do Field Sobriety Tests Prove?

The short answer is not much. At best, they may correlate to someone having a blood alcohol concentration over a .08. At worst, they prove nothing at all. To understand their meaning you must look at how they came into existence and who developed them.

In the late 1970’s, NHTSA (the National Highway Traffic and Safety Administration) paid for some research to see if tools could be developed for law enforcement to identify people who are potentially DUI / DWI.

There were several studies paid for by the government. Specifically, there are six primary studies relied on by law enforcement. However, none of the studies have been subject to peer review.

1977 Study (Not Peer Reviewed)

1981 Study (Not Peer Reviewed)

1983 Study (Not Peer Reviewed)

Colorado Study (Not Peer Reviewed)

Florida Study (Not Peer Reviewed)

San Diego Study (Not Peer Reviewed)

According to Wikipedia, peer review has been defined as:

...the process of subjecting an author's scholarly work, research, or ideas to the scrutiny of others who are experts in the same field. Peer review requires a community of experts in a given (and often narrowly defined) field, who are qualified and able to perform impartial review...Pragmatically, peer review refers to the work done during the screening of submitted manuscripts and funding applications. This process encourages authors to meet the accepted standards of their discipline and prevents the dissemination of irrelevant findings, unwarranted claims, unacceptable interpretations, and personal views. Publications that have not undergone peer review are likely to be regarded with suspicion by scholars and professionals.

Thus, the research has not been subject to scrutiny in the scientific community and begs the question why not? Only the government can answer this question.

So what does the government say these studies show? Unfortunately, much of law enforcement does not even know what their own research says. If you ask most officers who regularly conduct DUI investigations, they will tell you that field sobriety tests show “impairment.”

However, the government’s own research concludes this is not the case. If you are willing to accept these non-peer reviewed studies, then you may merely conclude that a poor performance correlates to a blood alcohol concentration above a .08. Moreover, the tests were previously used to show a blood alcohol concentration above a .10. Then the law changed in several jurisdictions, and somehow it also changed scientific results.

Consequently, even if you believe NHTSA’s own research, these agility tests do not show driving impairment.

This Video Demonstrates True Field Sobriety Testing

In case you ever wondered what happens on the side of the road when a person is stopped for DUI, here is a video showing exactly what goes on.  Enjoy:


DUI Stop from konu on Vimeo.

Imaginary DUI?

When police officers are attempting to determine whether to arrest someone for driving under the influence of alcohol (DUI), they usually ask them to perform some field sobriety tests.  Police generally rely on a battery of tests recommended by the National Highway Traffic Safety Administration (NHTSA).  One of these tests is referred to as the "Walk-and-Turn" test.  The instructions for performing the Walk-and-Turn test are contained in the NHTSA DWI Detection and Standardized Field Sobriety Testing Manual.

In essence, the Walk-and-Turn test requires that a person walk a straight line, touching heel to toe, and then turn around and walk back.  However, it seems many officers in Arizona (and apparently in other states as well) believe that using an imaginary line is a fair test.  That is, instead of having a person walk on a true line (i.e. a painted line in a parking lot) they have the person imagine a line to walk on.  Just this morning I was debating this issue with a Scottsdale Police Officer in a DUI case.

Austin, Texas DWI lawyer Jamie Spencer has written an excellent article entitled, Walking A Straight Line, arguing the merits of using an imaginary line during a Walk-and-Turn test.  He writes:

One common misconception that I see over and over on the part of officers, up to and including some of the local Austin DWI task force officers, is the belief that the book doesn’t require that a designated actual line be used.

If asked why the defendant was asked to walk an imaginary line instead of an actual line, most officers reply – some smugly – that ‘the manual’, that is the NHTSA manual, doesn’t require it. Some offer to show the defense lawyer exactly where in the book it says they don’t have to use an actual line.

Invariably, the officer will flip the pages and find this portion of the manual:

Procedures for Walk and Turn Testing

1. Instructions Stage: Initial Positioning and Verbal Instructions

For standardization in the performance of this test, have the suspect assume the heel-to-toe stance by giving the following verbal instructions, accompanied by demonstrations:

“Place your left foot on the line” (real or imaginary). Demonstrate.

[I’m taking this from the February 2006 Edition, Student Manual, page VIII-9 from Session VIII: Concepts and Principles of the Standardized Field Sobriety Tests. It should be in Chapter 8 of most or all other manuals.]

That certainly looks at first blush as if the manual says there’s no difference between the difficulty between walking an actual line, or walking an imaginary line. Although, it literally begs the question, “Officer, how wide a line did my client imagine?”

But no. The officer who so testifies is wrong. (Sorry, WJ, you’re wrong too.)

Flip the page once more – VIII-11 in the one I’m reading now - and you come to the part entitled:

4. Test Conditions

Walk-and-Turn test [sic] requires a designated straight line, and should be conducted on a reasonable dry, hard, level, nonslippery surface.

Requires. So, how to explain the seeming discrepancy? Easy.

The first section is talking about “Verbal Instructions” and is clearly labeled so. It is the Instructions Stage. That means… it is talking about the portion of the test where the officer demonstrates the Walk and Turn to the suspect.

So, going by the book, it’s perfectly OK for the officer to show the defendant how to do the test on his own imaginary line if he wants to do it that way. Heck, we all know they don’t even have to demonstrate all 9 steps. They are allowed to do it that way.

But the NHTSA Manual makes no bones about it: if this test is going to be administered properly, then the defendant is supposed to be afforded the opportunity to do it on an actual line. It is literally: required. And yes, that’s a potentially reasonable explanation for someone stepping ‘off the line’ – it wasn’t there in the first place.

Jamie's point cannot be overstated:  How can the officer say you stepped off a line that he cannot see?  After all, the line is in your head.  The officer does not know how long the line you imagined was; nor does the officer know how wide your imaginary line was.  Moreover, when a real line is available (such as in a parking lot), isn't it just common sense to use it during the test?