THE FOURTH Amendment protects Americans against “unreasonable searches and seizures.’’ But that protection, the Supreme Court has held consistently for more than 40 years, applies only where there is “a legitimate expectation of privacy.’’ This means, for example, that the police do not need a warrant to follow a suspect as he drives through town, since a person’s public movements are visible to anyone who chooses to look.

But that test is fast becoming outmoded in an age when advancing technology makes possible a degree of relentless 24/7 surveillance that would have been the stuff of science fiction four decades ago. The Supreme Court should take the first opportunity to reassess its test in light of improved electronic surveillance devices, and Congress should step in to provide guidelines of its own.

The latest case involves Juan Pineda-Moreno, an Oregon resident suspected by the federal Drug Enforcement Agency of marijuana trafficking. Sneaking onto his property without a warrant late one night, DEA agents attached a GPS tracking device to his Jeep, which was parked in the driveway. Weeks later — alerted by the GPS monitor — the agents pulled Pineda-Moreno over as he was driving out of a suspected marijuana growing site. Sure enough, his Jeep contained a hefty stash of marijuana.

A three-judge panel of the Ninth Circuit ruled that the agents had done nothing wrong in planting the GPS tracker, since there is no reasonable expectation of privacy in an open driveway. After all, the judges reasoned, no one would object “if a neighborhood child had walked up Pineda-Moreno’s driveway and crawled under his Jeep to retrieve a lost ball or runaway cat.’’

Is it only the well-to-do whose cars — secure in underground garages or behind tall walls or electric gates — are protected by the Fourth Amendment?

Other courts have come to a different conclusion. Last month the DC Circuit ruled that long-term GPS monitoring is a search under the Fourth Amendment. The issue should ultimately end up before the Supreme Court. But Congress need not wait. It can act now to impose reasonable guidelines on the use of high-tech surveillance without a warrant or probable cause. Several states have already done so. Last year, the Supreme Judicial Court of Massachusetts ruled that under Article 14 of the Massachusetts Declaration of Rights, the use of a GPS tracking device is a “seizure’’ and therefore requires a warrant.