The Fourth Amendment protects against "unreasonable searches." The word "unreasonable" is unavoidably vague and necessarily requires the courts to make value judgments. However, judging after the fact whether someone's actions met a vaguely defined standard can be unfair to the person being judged, and it provides inadequate protection against future violations. A person cannot be deterred from crossing a line if does not know where the line is. So, as Justice Souter wrote for the Court in Atwater v. City of Lago Vista, 532 U.S. 318, 347 (2001),

"we have traditionally recognized that a responsible Fourth Amendment balance is not well served by standards requiring sensitive, case-by-case determinations of government need, lest every discretionary judgment in the field be converted into an occasion for constitutional review. See, e. g., United States v. Robinson, 414 U. S. 218, 234-235 (1973). Often enough, the Fourth Amendment has to be applied on the spur (and in the heat) of the moment, and the object in implementing its command of reasonableness is to draw standards sufficiently clear and simple to be applied with a fair prospect of surviving judicial second-guessing months and years after an arrest or search is made."

Bright-line rules, like presumptions, need to be crafted so that they get us to the same result most of the time that a case-by-case analysis would reach. New York v. Belton, 453 U.S. 454 (1981) held that a lawful custodial arrest of the occupant of a car authorized a search of the car as well. Given that officers could search the car if (1) a protective sweep was needed for their own safety or (2) there was probable cause that evidence of the crime would be in the car or (3) they were going to impound the car, authorizing an inventory search, there are probably relatively few cases where a search would not be authorized on a case-by-case review. Thornton v. United States, 541 U.S. 615 (2004), extended Belton to a situation where the driver had left the car before he was arrested, and he was handcuffed and in the back of the squad car. Justice Scalia, joined by Justice Ginsburg, concurred in the judgment on the ground that there was probable cause in the individual case, but he believed "[t]he Court's effort to apply our current doctrine to this search stretches it beyond its breaking point...."

Gant is much like Thornton. The officer safety rationale is thin, to put it mildly. Justice Kennedy tried to pry an additional rationale out of the attorneys for the State and the United States, without much success. Justice Breyer, who was in the majority in Thornton, and Justice Alito, who was not on the Court at the time, asked Gant's attorney about stare decisis (misspelled "decisus" throughout the transcript). If the state wins this case, that may be the key. Originally justified or not, Belton has been the rule for 27 years, and it has not proven unworkable or caused widespread abuses. Defense counsel might have done better by taking a firm position that the venerable Belton can stay but newcomer Thornton has to go.

The docket shows that Arizona had amicus support from the United States, Florida and other states, Los Angeles County, the National Association of Police Organizations, and Americans for Effective Law Enforcement. CJLF passed. Personally, I'm inclined to agree with Justice Scalia on this one. Officer safety is certainly a weighty consideration, and our brief defends a broad "frisk" authority on that basis in the other Arizona Fourth Amendment case, Arizona v. Johnson, No. 07-1122, but I don't see that this situation genuinely presents that risk.

For other views on the argument, see Kevin Russell at SCOTUSblog on both Gant and Herring, and Orin Kerr at VC. The latter notes, "If you are a fan of the popular meme of 'our Supreme Court is filled with conservative activists who want to take away our rights,' you may want to avoid this case to minimize uncomfortable cognitive dissonance."