Flashing Lights and HGN in Texas DWI Cases: What the Manual Actually Says

Flashing lights are a question to investigate, not an automatic result

The presence of emergency lights does not, by itself, establish that an HGN test is invalid or that a DWI arrest was unlawful. HGN, or horizontal gaze nystagmus, is an eye-movement evaluation. Review the actual administration rather than drawing a conclusion from a light bar appearing in the recording.

NHTSA’s 2023 SFST Participant Manual discusses optokinetic nystagmus from watching certain moving visual images. It also explains that properly administered HGN is not influenced by that form of nystagmus because the subject fixates on the moving test stimulus. Both parts of that explanation matter.

Reconstruct the administration from the recording

Identify where the driver and officer stood, what the driver was asked to follow, whether the eyes remained on that stimulus and whether an interruption occurred. Compare the spoken instructions and sequence with the report.

A body camera does not necessarily show exactly what the driver could see. Reflections and a camera’s exposure settings do not measure a particular physiological response. If the footage cannot resolve a detail, identify the gap rather than claim that it proves contamination.

Separate technique, admissibility and weight

A lawyer can evaluate the witness’s qualifications, the technique used and its application in the case. A training concern may support further investigation, a legal objection or cross-examination, depending on the record and governing law.

Neither an alleged deviation nor a vivid video guarantees suppression, dismissal or reversal. The court’s ruling and the significance of other evidence must be assessed separately. HGN also should not be presented as a precise numerical BAC measurement.

Prepare a useful account

Tell your lawyer accurately about visibility, instructions, interruptions and any relevant eye condition. Existing medical information may help frame a question, but a diagnosis alone does not prove why an observation occurred.

The goal is to identify a specific supported concern. Avoid replacing the officer’s assumption with an equally unsupported claim that all flashing lights create a false positive.

Questions and records to compare

Question	Useful record	Limit to remember
What was watched?	Instructions and video	Light in the frame does not prove loss of fixation
How was HGN performed?	Sequence, report and qualifications	An alleged deviation is not automatic exclusion
What can video establish?	Available camera views	Camera viewpoint and driver viewpoint can differ

What to bring

• Police report and available video information

• Your recollection of instructions and interruptions

• Existing relevant eye or medical records

• Questions about camera gaps

• Charging and bond documents

Bring what you already have. Ask your lawyer about obtaining missing material, and do not post private case details in public comments.

Questions about your Houston DWI case?

To discuss representation, call Ruben Restrepo at 713-714-7330 or contact the office. Learn more about our Houston DWI practice.

Sources and review

Reviewed October 20, 2026. General information; the facts, applicable law, notices and orders in an individual case matter.

• NHTSA/IACP — 2023 SFST Participant Manual — Revised February 2023. Local primary PDF reviewed: Session 7, pp. 18–23 / PDF pp. 203–208 (PBT); Session 8, p. 16 / PDF p. 229 (optokinetic nystagmus). Training guidance is not a Texas exclusion rule.