The Limits of SFSTs in a Drug DUI Prosecution

Three field tests are offered to a Georgia jury in nearly every contested DUI: Horizontal Gaze Nystagmus, the Walk-and-Turn, and the One-Leg-Stand. In a drug case, all three are being asked to perform work they were never designed — or validated — to do.

I · OriginWhat the NHTSA studies actually established

The Standardized Field Sobriety Test battery emerged from work conducted by the Southern California Research Institute in the late 1970s and early 1980s under contract with the National Highway Traffic Safety Administration. The validation studies — Burns & Moskowitz (1977), Tharp, Burns & Moskowitz (1981), and the San Diego field validation (Stuster & Burns, 1998) — were designed and conducted to identify drivers with blood alcohol concentrations at or above a defined threshold. Drugs were not the subject of those studies. They are not the subject of the validation today.

II · HGNWhy nystagmus is the wrong measurement for cannabis

Horizontal Gaze Nystagmus is a measurement of involuntary jerking of the eye as it tracks a stimulus to the periphery. The medical literature is consistent on this point: HGN is produced by central nervous system depressants — alcohol, benzodiazepines, barbiturates, certain inhalants — and by a number of medical conditions unrelated to intoxication, including fatigue, caffeine, and natural physiological nystagmus.

Marijuana is not a CNS depressant in the same class. The DRE protocol itself, in its standard reference materials, categorizes cannabis as not producing horizontal gaze nystagmus. A Georgia officer who testifies to nystagmus clues in a marijuana-only case is offering evidence the protocol does not support.

III · Divided AttentionWalk-and-Turn and One-Leg-Stand at roadside

The divided-attention tests are scored on standardized clues — eight for the Walk-and-Turn, four for the One-Leg-Stand. NHTSA's validation studies report accuracy rates for alcohol of roughly 79 percent and 83 percent respectively, but only when administered in strict compliance with the standardized procedures. That qualifier is doing a great deal of work.

• Uneven pavement, gravel, sloped shoulders, and roadside debris compromise the surface assumptions of the validation studies.
• Footwear — boots, heels, sandals — is rarely accounted for.
• Age, weight, prior injury, inner-ear conditions, and recent physical activity all affect performance independent of any impairing substance.
• The instructions are dense, and on body-worn camera the suspect's confusion is often visible long before the test begins.

IV · Cross-ExaminationWhat a careful officer will concede

A well-trained officer, examined patiently and without confrontation, will concede:

1. The tests were validated for alcohol detection.
2. The studies do not establish a comparable accuracy rate for cannabis.
3. Cannabis is not listed as a HGN-producing substance in the standard protocol.
4. Any deviation from standardized administration affects the reliability of the indicator.
5. The clues are indicators, not measurements.

Each concession is unremarkable. Cumulatively, they reframe the State's case as what it actually is — a structured set of observations offered to support an inference of impairment — and give the jury permission to ask whether that inference has been established beyond a reasonable doubt.

Cross-examination is not a fight. It is the patient construction of a record the jury will trust more than the witness.

V · The Trial LessonRestraint, not theatrics

DUI-drugs cases are not won by impeaching the officer's character or by aggressive performance at the lectern. They are won by isolating, in plain language, the difference between what the battery was designed to detect and what the State has charged the defendant with. A Georgia jury that understands that difference understands the case.

VI · Related Reading

• Defending Marijuana DUI Allegations in Georgia
• Cross-Examining a Toxicologist in a Georgia DUI-Drugs Trial
• THC Blood Testing in Georgia: GBI Methodology
• Polysubstance DUI Defense in Georgia
• Georgia DUI-Drugs Defense — Practice Overview

This analysis is general in nature and does not constitute legal advice. Prior results do not guarantee a similar outcome.