Each defendant was arrested at a permanent checkpoint operated by the Border Patrol away from the international border with Mexico, and each sought the exclusion of certain evidence on the ground that the operation of the checkpoint was incompatible with the Fourth Amendment. ''Approximately one mile south of the checkpoint is a large black-on-yellow sign with flashing yellow lights over the highway stating 'ALL VEHICLES, STOP AHEAD, 1 MILE.' Three-quarters of a mile further north are two black-on-yellow signs suspended over the highway with flashing lights stating 'WATCH FOR BRAKE LIGHTS.' At the checkpoint, which is also the location of a State of California weighing station, are two large signs with flashing red lights suspended over the highway. These signs each state 'STOP HERE -- U.S. OFFICERS.' Placed on the highway are a number of orange traffic cones funneling traffic into two lanes where a Border Patrol agent in full dress uniform, standing behind a white-on-red 'STOP' sign checks traffic. Blocking traffic in the unused lanes are official U.S. Border Patrol vehicles with flashing red lights. In addition, there is a permanent building that houses the Border Patrol office and temporary detention facilities. There are also floodlights for nighttime operation.'' Agents stand between the two lanes of traffic visually screening all northbound vehicles. Most motorists resume their progress without any oral inquiry or close visual examination. Some cars are directed to a secondary inspection area, where their occupants are asked about their citizenship and immigration status. D approached the checkpoint driving a vehicle containing two female passengers. The women were illegal Mexican aliens. They used false papers and rendezvoused with D in San Diego to be transported northward. D's car was directed to the secondary inspection area. D produced documents showing him to be a lawful resident alien, but his passengers admitted to being present in the country unlawfully. D was charged with two counts of illegally transporting aliens. D’s motion to suppress claiming the checkpoint violated the 4th Amendment was denied and he was convicted. The court of appeals held that the stop violated the Fourth Amendment, concluding that a stop for inquiry is constitutional only if the Border Patrol reasonably suspects the presence of illegal aliens on the basis of articulable facts. P appealed.