Police in New York City have begun randomly searching the bags of subway riders for explosives. Since they will be able to check only a small percentage of bags, the main point of the policy seems to be creating the appearance of safety:
William W. Millar, president of the American Public Transportation Association, an industry group, said comprehensive coverage of any major urban transit system would be next to impossible. "If you were going to try to check a very high percentage at every station or on every train, it would be incredibly labor-intensive," he said.
Still, he said, the searches could deter would-be attackers and improve the public's confidence. "The public wants to feel safe, as well as be safe," he said. "So this has a benefit of perception."
Is the benefit of perception worth the cost of privacy? Police say anyone who doesn't want to open his bag won't be allowed on the subway but will otherwise be free to go. If you "consent" to a search, however, you will be subject to arrest should the police find drugs, an illegal weapon, or (presumably) any other contraband. That secondary purpose makes it less likely that the searches will pass constitutional muster, since they are not aimed simply at preventing terrorist attacks (or even creating the illusion of preventing terrorist attacks).