State v. Justin Keith Austin, 413 P.3d 778

Background

Shortly after midnight on April 5, 2015, Deputy Richardson stopped Justin Austin for failing to signal; he smelled alcohol, observed bloodshot eyes, and Austin admitted drinking three drinks shortly before driving.
Austin failed field sobriety tests and, ~30 minutes after the stop, submitted to two breath tests registering .085 and .086 BAC.
The State charged Austin under Idaho Code § 18-8004 both for impairment and under the per se theory (BAC ≥ .08).
Austin sought to admit expert extrapolation testimony (to show his BAC was ~.06–.065 while driving and rose to >.08 by testing) based on rapid consumption just before the stop.
The district court granted the State’s motion in limine excluding expert testimony that would extrapolate Austin’s BAC at the time of driving as irrelevant to the per se theory, though it allowed such evidence for the impairment theory; Austin declined to call his expert and was convicted.
The Idaho Supreme Court vacated the conviction, holding the exclusion of extrapolation evidence as to the per se charge was an abuse of discretion and not consistent with clarified legal standards.

Issues

Issue	Plaintiff's Argument	Defendant's Argument	Held
Admissibility of expert extrapolation evidence for per se DUI	State: Per Elias‑Cruz/Tomlinson, only the BAC at time of test matters; extrapolation to driving time is irrelevant	Austin: Extrapolation is relevant to whether he was over .08 while driving; excluding it limits his right to present a defense	Court: Exclusion was an abuse of discretion; defendants may present extrapolation evidence to challenge per se proof
Whether per se statute requires BAC ≥ .08 while driving	State: Statute does not require proving BAC was ≥ .08 while driving; a qualifying analysis showing ≥ .08 suffices	Austin: Per se theory should be tied to BAC at the time of driving; otherwise defendants cannot rebut state evidence	Court: Statute allows state to rely on test results without extrapolation in its case‑in‑chief, but that does not render a defendant’s contrary evidence irrelevant; nexus to driving remains required
Harmless error from excluding expert testimony	State: Error harmless because Austin could be convicted under impairment theory	Austin: Exclusion affected substantial rights; jury instructions covered both theories so harmlessness not established	Court: Error was not harmless because jury’s basis for conviction is unclear; exclusion impacted substantial rights
Constitutional vagueness of statute (time limitation for testing)	Austin: Lack of a time limit for admissible tests makes statute vague/overbroad	State: Prosecutorial discretion and statutory language suffice	Court: Did not reach constitutional question because case resolved on evidentiary grounds

Key Cases Cited

Elias‑Cruz v. Idaho Dep’t of Transp., [citation="153 Idaho 200, 280 P.3d 703"] (2012) (per se proof may rest on statutory evidentiary tests; lapse of time goes to weight, not admissibility)
State v. Tomlinson, [citation="159 Idaho 112, 357 P.3d 238"] (Ct. App. 2015) (court of appeals held state need not extrapolate test results back to driving and suggested driving‑time BAC is irrelevant)
State v. Robinett, [citation="141 Idaho 110, 106 P.3d 436"] (2005) (lapse‑of‑time principle: test timing affects weight of evidence)
State v. Faught, [citation="127 Idaho 873, 908 P.2d 566"] (1995) (standard of review for admission of expert testimony)