What is the test for probable cause to search a vehicle following a traffic stop?

The crucial issue is the legality of the original stop. In the present case, there was a clear government interest in preventing and detecting the illegal entry of aliens into the United States. A checkpoint had been set up on Interstate 5, a major north-south highway extending to the Mexican border, at a location where transportation of aliens who had illegally entered the United States was known to occur. The officer had observed an older model car at 1:40 a. m., which appeared to be carrying a heavy load in the rear. We have held that it is constitutionally permissible to detain persons briefly under circumstances not justifying an arrest for purposes of limited inquiry in the course of routine police investigation. United States v. Oswald, 441 F.2d 44, 46 (9th Cir. 1971). From that point on, probable cause to search developed rapidly and indisputably.²