Cite as: 534 U. S. 266 (2002)
Opinion of the Court
fic on 191 as part of a coordinated effort to stem the flow of illegal immigration and smuggling across the international border. See id., at 20-21. Agents use roving patrols to apprehend smugglers trying to circumvent the checkpoint by taking the backroads, including those roads through the sparsely populated area between Douglas and the national forest. Id., at 21-22, 26, 80. Magnetic sensors, or "intrusion devices," facilitate agents' efforts in patrolling these areas. See id., at 25. Directionally sensitive, the sensors signal the passage of traffic that would be consistent with smuggling activities. Ibid.; Tr. of Oral Arg. 23-24.
Sensors are located along the only other northbound road from Douglas besides Highways 191 and 80: Leslie Canyon Road. Leslie Canyon Road runs roughly parallel to 191, about halfway between 191 and the border of the Coronado National Forest, and ends when it intersects Rucker Canyon Road. It is unpaved beyond the 10-mile stretch leading out of Douglas and is very rarely traveled except for use by local ranchers and forest service personnel. App. 26. Smugglers commonly try to avoid the 191 checkpoint by heading west on Rucker Canyon Road from Leslie Canyon Road and thence to Kuykendall Cutoff Road, a primitive dirt road that leads north approximately 12 miles east of 191. Id., at 29-30. From there, they can gain access to Tucson and Phoenix. Id., at 30.
Around 2:15 p.m., Stoddard received a report via Douglas radio that a Leslie Canyon Road sensor had been triggered. Id., at 24. This was significant to Stoddard for two reasons. First, it suggested to him that a vehicle might be trying to circumvent the checkpoint. Id., at 27. Second, the timing coincided with the point when agents begin heading back to the checkpoint for a shift change, which leaves the area unpatrolled. Id., at 26, 47. Stoddard knew that alien smugglers did extensive scouting and seemed to be most active when agents were en route back to the checkpoint. Another border patrol agent told Stoddard that the same
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