United States v. Potter, 78 F.4th 486

In May 2021 a Hooksett, NH officer stopped a vehicle for an alleged failure to signal where the roadway narrows from two lanes to one; passenger Steven Potter was found to have outstanding warrants, arrested, and a bag of narcotics seized. Potter was charged with possession with intent to distribute.
Potter moved to suppress the seized items, arguing New Hampshire law (N.H. Rev. Stat. § 265:45) does not require a turn signal at the roadway "merge" point, so the stop lacked probable cause or reasonable suspicion.
The district court held the statute requires signaling only for turning, changing lanes, or starting from a parked position and therefore does not require a signal at a lane "merge" where two lanes blend into one; it granted suppression and found the statute unambiguous.
The government appealed, arguing the stop was nevertheless reasonable because the officer made an objectively reasonable mistake of fact (relying on a posted "Lane Ends" sign) or, alternatively, a reasonable mistake of law about the signaling requirement.
The First Circuit affirmed: it reviewed law de novo and facts for clear error, concluded the officer’s mistake of fact was not objectively reasonable because the physical roadway contradicted the sign and the officer had direct visual familiarity, and therefore affirmed suppression without resolving the mistake-of-law claim.

Issue	Plaintiff's Argument (Government)	Defendant's Argument (Potter)	Held
Whether NH law required a turn signal at the narrowing/merge point	(did not contest on appeal)	Statute does not require signaling when two lanes blend into one	District court and court of appeals: statute does not require signal at that merge point
Whether the stop was justified by an objectively reasonable mistake of fact (relying on the "Lane Ends" sign)	Officer reasonably relied on posted sign to conclude right lane ended and a lane change (thus signaling) was required	The posted sign conflicted with the actual roadway; officer observed the blend and was familiar with it, so reliance on the sign was unreasonable	Mistake of fact was not objectively reasonable; stop was unlawful
Whether a reasonable mistake of law could justify the stop	Officer reasonably (but mistakenly) believed the law required a signal at that location	Any mistake of law is not rescuing because the underlying factual mistake was unreasonable	Court declined to decide mistake-of-law because mistake-of-fact failed; suppression affirmed

Heien v. North Carolina, 574 U.S. 54 (2014) (Fourth Amendment searches and seizures can be reasonable when based on an objectively reasonable mistake of fact or law)
Brendlin v. California, 551 U.S. 249 (2007) (occupants of a stopped vehicle are seized under the Fourth Amendment)
Whren v. United States, 517 U.S. 806 (1996) (probable cause for a traffic violation justifies a stop)
United States v. Moran, 944 F.3d 1 (1st Cir. 2019) (objective-reasonableness inquiry considers facts available to officers at time of stop)
United States v. Reyes, 24 F.4th 1 (1st Cir. 2022) (standard of review for suppression rulings and discussion of reasonable suspicion/probable cause for traffic stops)
United States v. Orth, 873 F.3d 349 (1st Cir. 2017) (review standards for suppression rulings)
United States v. Miles, 18 F.4th 76 (1st Cir. 2021) (traffic-stop reasonable-suspicion discussion involving signage)
United States v. Zannino, 895 F.2d 1 (1st Cir. 1990) (argument or issue not raised is waived)