What is sufficient reasonable suspicion to stop a vehicle for a brief investigatory stop?

Law enforcement officers "may make a brief investigatory stop of a moving vehicle ... if under the totality of the circumstances, they are aware of articulable facts leading to a reasonable or founded suspicion that the person has been, is, or is about to be engaged in criminal activity." United States v. Fouche, 776 F.2d 1398, 1402 (9th Cir.1985). Traffic violations constitute criminal conduct and provide sufficient reasonable suspicion for an investigatory stop. Id. at 1403; see also United States v. Perez, 37 F.3d 510, 513 (9th Cir.1994) (sufficient reasonable suspicion existed for a stop when the driver weaved back and forth across the fog line); cf. Millan, 36 F.3d at 889-90 (a minor crack in the windshield may not provide sufficient reasonable suspicion to stop a vehicle).