But even with the new legal authority to set up frequent checkpoints, it was not until 2006 that their extensive use became evident.

Search and Seizure

“It is assumed under the law that the taking of personal property without a warrant is unconstitutional,” said Martin J. Mayer, a founding partner in the Fullerton law firm Jones & Mayer, who represents numerous law-enforcement associations.

Mr. Mayer was referring to the Fourth Amendment to the United States Constitution, which restricts law enforcement’s authority to seize private property without a court order. That, he said, protects everyone, whether they are in the country legally or not. He communicated his opinion in a memorandum to his clients, California’s police chief and sheriffs, in 2005 after a ruling by the Ninth Circuit in an Oregon case.

In that decision the court held that law enforcement could not impound a vehicle if the only offense was unlicensed driving. The only exception permitted the police to impound a car was if it posed a threat to public safety, was parked illegally or would soon be vandalized if left in place.

The ruling sharply altered the legal status of vehicle impoundments. In response, in 2007 the legislative counsel of California called on the state’s police departments to cease what had become a standard practice.

“If a peace officer lawfully stops a motor vehicle on the highway and the driver of the motor vehicle is an unlicensed driver, that alone is not sufficient justification for the peace officer to cause the impoundment of the motor vehicle,” the counsel, who advises lawmakers, said in a report. But the counsel has no authority over police departments.

In 2008, in a separate case in Federal District Court case, a judge arrived at a different conclusion, agreeing with the State of California and several cities, who were defendants and argued that the impoundments were penalties for a criminal offense, and that therefore car owners were not subject to Fourth Amendment protections. The plaintiffs’ appeal is pending.