The following excerpt is from U.S. v. Chico, 977 F.2d 592 (9th Cir. 1992):
The fact that the turn signal of the appellees car remained on after the car turned and the appellees' car proceeded straight after beginning a left turn also fails to justify an investigative stop. Minimal deviations from normal driving in reaction to a marked patrol car do not demonstrate founded suspicion of criminal activity. See United States v. Robert L., 874 F.2d 701, 704 (9th Cir.1989) (leaving the scene of traffic accident more quickly than other traffic and swerving into another lane was a reasonable response when a marked patrol car in the lane behind the defendant approached at high speed). An innocent driver might also slow down at seeing a marked patrol car on a deserted highway and then drive in a mildly unusual manner either in reaction to being followed or because he was in an unfamiliar town late at night.
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