By Aaron Botts
It is easy to decide to arrest a driver who is obviously impaired. The harder work, and the work that separates true experts, is making the right arrest decision when impairment is subtle, and the driver is just above the statutory limit. That is exactly where SFST administration matters most. Small errors do not just create testimony problems; they reduce sensitivity, increase false negatives, and can allow impaired drivers to keep driving.
The standard matters because your decision depends on it SFSTs are a standardized way to simulate divided attention and observe impairment in a repeatable, evidence-based way. The closer your administration stays to the standardized method, the more accurate your decision is likely to be, especially near the limit. Below are three common administration mistakes, one from each test, and how to fix them so your roadside decision making improves.
- HGN, Lack of Smooth Pursuit: inconsistent stimulus speed The mistake: Slowing down, pausing, or changing speed as you cross the midline while checking for Lack of Smooth Pursuit. Why it hurts accurate decision making Lack of Smooth Pursuit is about the eyes tracking a moving stimulus. If you slow down or stop in the middle, you make it harder to detect true pursuit problems and you can increase the chance of false negatives. Put simply, you may miss nystagmus that is actually there because your stimulus movement is not steady. How to do it correctly Dr. Karl Citek, O.D., Ph.D., in his presentation, “Intoxication vs. Medical Condition,” has described proper movement for the smooth pursuit check as approximately 30 degrees per second, which works out to about 4 seconds to move the stimulus across the full field of vision from one maximum deviation to the other, maintaining a consistent speed through the center. Sound familiar? A practical tool that helps officers stay consistent is this: imagine a long rubber band anchored to the wall on your left and looped around your index finger. As you move your stimulus to the subject’s maximum deviation of the left eye and then sweep across the field of vision, keep the “tension change” constant as the rubber band steadily pulls your finger all the way to the subject’s maximum deviation of the right eye. No speed changes. No midline pauses.
Dr. Citek also reminds us that occasional errors in a subject’s tracking ability can occur, particularly in the center of the vision, called a midline jump, or with people generally over 70 years of age having a decreased gain, meaning their eyes cannot track the stimulus that quickly. This is where the bonus tip at the end of this article is paramount.
Bottom line Clean, consistent movement increases your ability to detect Lack of Smooth Pursuit. Inconsistent movement increases false negatives and reduces your confidence in what you are seeing.
- Walk and Turn: re-explaining instead of answering the question The mistake: Re-teaching or re-demonstrating the whole Walk and Turn when the subject asks one simple question. Why it hurts accurate decision making Part of the Walk and Turn test is the subject’s ability to listen, retain instructions, and divide attention while balancing. When you re-explain the entire test, or large portions of the test, you make it easier than designed. That matters most with borderline drivers. A person who is just above the limit may look significantly better after being coached, which can push you toward a false negative. How to do it correctly Give the standardized instructions once. If you give the instructions exactly as administered in the manual,(Session 8: p.52-53) you will say each instruction only once, except for the instruction about nine steps, which you say two times for each set of steps. If the subject says he understands, let him perform. If he asks a question, answer only that question and return to the instructions without restarting the entire sequence. Example: • Subject: “How do I turn?” • Better: “Keep your lead foot on the line and take a series of small steps to turn. Do you understand?” • Do not redo the whole demonstration unless he specifically asks, or you truly need to restart because the subject is not tracking at all.
Bottom line Do not turn a divided attention test into a coached performance. Answer the question asked, then run the test.
One Leg Stand: repeating the instructions because of uncertainty or habit The mistake: Repeating the instructions multiple times. Why it hurts accurate decision making The One Leg Stand test also pressures divided attention and short-term memory. Repeating instructions makes the test easier, which again matters most near the limit. If a borderline driver performs better because you repeated and reinforced the instructions, you have reduced the test’s sensitivity and made your arrest decision harder. How to do it correctly Deliver the instructions cleanly and start the test once the subject confirms he understands. If you give the instructions exactly as administered in the manual (Session 8: p.62-63), the only instruction you repeat is to keep the arms at the sides. If a question is asked, answer it and proceed. If you repeat anything, keep it identical, unless they are not understanding a certain word – like parallel. Bottom line Trust the standardized script. Do not lengthen the instruction phase out of habit, uncertainty, or because others do it that way. Bonus Tip: Use the PBT to improve decision accuracy If your policy and law allow PBT use, it is one of the best tools to improve the accuracy of your roadside arrest decision when combined with behavioral testing. A NHTSA field evaluation (Anderson & Snyder, 1983) reported:
Agencies using the sobriety test battery alone had 83% of arrests at 0.10% BAC and above.
PBT-only was 89%
The combination of the battery plus PBT was 90%.
The researchers found the use of a PBT alone, or the use of the SFSTs and a PBT, was “far superior when compared to the normal DWI arrest procedure.”
Even if PBT results are not admissible in your courtroom, the point here is decision quality.
Final Thought If you want to be better at catching the driver who is just above the limit, you need the tests to work the way they were designed to work. Clean HGN movement, disciplined instruction delivery, and avoiding accidental coaching will improve your accuracy and reduce false negatives. Add a PBT when available, and you further improve decision quality. The goal is simple: fewer missed impaired drivers and fewer avoidable errors that make the arrest decision harder.
- Anderson & Snyder, Field Evaluation of a Behavioral Test Battery for DWI, DOT HS 806 475 (NHTSA Sept. 1, 1983)
