For me, last week was a vacation week, spent in San Diego. My family reaches that city on Interstate 8, which lies in what the American Civil Liberties Union calls the "Constitution-free zone" within 100 miles of the United States border. This trip, that meant not one, but two stops at Border Patrol checkpoints (PDF). This time, we had to assure officials of our citizenship at permanent checkpoints instead of at the roving "tactical" checkpoints that appear hither and yon, not that there's much difference from motorists' point of view. After the second stop in the middle of the desert, surrounded by armed men and "working dogs" (as the signs assured us), my wife turned to me and said, "we need to start carrying Tony's passport when we're driving."
Tony is seven, and surrounded though he is in the back seat by Calvin and Hobbes compilations and the latest book in the N.E.R.D.S. series that he's taken to, he has little in the way of legal documentation. True, no Border Patrol officer at an interior checkpoint has yet insisted that I provide proof of his identity, let alone his citizenship, but a stretch of blazing desert highway halfway between wide spot in the road and dead coyote is no place to be surprised. Hence, my wife's new conviction that we really need to start carrying our son's passport so we can display it on demand while traveling within the United States, without ever crossing an international border.
U.S. Customs and Border Protection insists (PDF) that "[a]ll Border Patrol checkpoints operate in accordance with the Constitution of the United States and governing judicial rulings," and they're correct, if you leave room for interpretation and some vigorous wiggling. The Supreme Court has ruled, in cases including United States v. Montoya de Hernandez, that "[a]utomotive travelers may be stopped at fixed checkpoints near the border without individualized suspicion, even if the stop is based largely on ethnicity." The executive branch has interpreted "near" to mean "100 miles" which, I guess, is near enough if you work on a sufficiently large cartographic scale.
The Government Accountability Office clarifies the official position a bit further (PDF):
Border Patrol agents at checkpoints have legal authority that agents do not have when patrolling areas away from the border. The United States Supreme Court ruled that Border Patrol agents may stop a vehicle at fixed checkpoints for brief questioning of its occupants even if there is no reason to believe that the particular vehicle contains illegal aliens. The Court further held that Border Patrol agents "have wide discretion" to refer motorists selectively to a secondary inspection area for additional brief questioning. In contrast, the Supreme Court held that Border Patrol agents on roving patrol may stop a vehicle only if they have reasonable suspicion that the vehicle contains aliens who may be illegally in the United States—a higher threshold for stopping and questioning motorists than at checkpoints.12 The constitutional threshold for searching a vehicle is the same, however, and must be supported by either consent or probable cause, whether in the context of a roving patrol or a checkpoint search.
By the way … "Probable cause?" That's where those "working dogs" come in. As Reason's Jacob Sullum has reported, dogs may not be reliable detectors of contraband, but they are reliable sources of probable cause.
That 100 mile strip is pretty well populated, being full of major cities and densely setted coastal states. The ACLU ran some calculations and reports, "What we found is that fully TWO-THIRDS of the United States' population lives within this Constitution-free or Constitution-lite Zone. That's 197.4 million people who live within 100 miles of the US land and coastal borders."
Most of those areas aren't subject to routine stops and status checks. That's a special joy generally reserved for residents of the Southwest and some parts of the country bordering Canada. But the federal government's position is that it can impose such checks within 100 miles of the border if it so chooses.
Maybe packing your kids' passports for travel within the United States isn't such a crazy idea. We already have the internal checkpoints; we might as well make sure we all have our papers in order to match.