State of Tennessee v. Linzey Danielle Smith, 484 S.W.3d 393

At ~3:00 a.m. on I‑65 Trooper Achinger observed Linzey Smith drift toward the right shoulder in a curve: once both right wheels crossed the fog line and twice the right wheels touched it, over ~0.4–0.5 mile; no other traffic was endangered.
The trooper followed about two more miles, videoed the drive (the camera did not capture the fog‑line crossings), and stopped Smith after she exited the interstate.
Smith was charged with DUI; she moved to suppress evidence arguing the stop violated the Fourth Amendment and Tenn. Const. art. I, § 7 because there was no probable cause or reasonable suspicion under Tenn. Code Ann. § 55‑8‑123(1).
The trial court denied the motion; Smith pleaded guilty reserving a certified question; the Court of Criminal Appeals affirmed and the Tennessee Supreme Court granted review.
The Supreme Court construed § 55‑8‑123(1) (the “as nearly as practicable” lane requirement), held that slight lane excursions can violate the statute depending on circumstances, and concluded the trooper had reasonable suspicion to stop Smith.
The Court affirmed the conviction, finding the stop constitutional (reasonable suspicion) though not supported by probable cause.

Issue	Plaintiff's Argument	Defendant's Argument	Held
1) What does Tenn. Code Ann. § 55‑8‑123(1) prohibit?	Smith: statute should be read to require a showing of unsafe conduct or danger before a violation is found.	State: statute requires drivers to remain within a lane "as nearly as practicable" and not move without ascertaining safety; lane departures can be violations without proof of actual danger.	Court: § 55‑8‑123(1) contains two distinct rules — (a) stay in a lane as nearly as practicable and (b) do not move lanes without first ascertaining safety; either element can be violated even if no actual danger occurred.
2) Was the traffic stop constitutional (probable cause or reasonable suspicion)?	Smith: trooper lacked probable cause and no reasonable suspicion existed given curve, road conditions and only brief crossings.	State: trooper observed crossings/touches of the fog line and reasonable factors (time, dry road, interstate) supported at least reasonable suspicion to investigate.	Court: no probable cause, but totality of circumstances (time of night, dry conditions, interstate curve but ample lane width, repeated crossings) gave reasonable suspicion to stop and investigate under § 55‑8‑123(1).

Terry v. Ohio, 392 U.S. 1 (1968) (authorizes brief investigatory stops on reasonable suspicion)
Whren v. United States, 517 U.S. 806 (1996) (probable cause for traffic violations justifies a stop regardless of officer motive)
Ornelas v. United States, 517 U.S. 690 (1996) (probable cause is a practical, nontechnical concept; review includes objective analysis)
State v. Brotherton, 323 S.W.3d 866 (Tenn. 2010) (Tenn. case applying reasonable‑suspicion analysis to investigatory traffic stop)
State v. Odom, 928 S.W.2d 18 (Tenn. 1996) (standard of review for suppression‑hearing factual findings)
State v. Day, 263 S.W.3d 891 (Tenn. 2008) (reasonable suspicion level is lower than probable cause; requires particularized, articulable facts)