by Christopher Dunn and Donna Lieberman — Picture yourself in an airport worried about terrorists boarding your plane. Imagine the airport has ten separate entrances to the boarding area, that the police have set up a checkpoint at just one of those entrances, and at that one checkpoint are checking the bags of only one out of every ten people, without regard to suspicion. Finally, imagine the police allow anyone selected for search at that one checkpoint to walk away, free to enter the boarding area through any of the remaining nine unguarded entrances. Would you feel safe? Call us loony, but we think that most people would not. Yet that's the system being used by the NYPD to protect the subway: police checkpoints at a small number of entrances where people are selected for search according to a numerical formula (like one out of every ten) and are allowed to walk away if picked for search. Under this program, tens of thousands of innocent New Yorkers trying to get to work, visit friends, or just move about town have been searched by the police without any suspicion of wrongdoing. Meanwhile, those bent on doing harm remain free to enter the system. Whether this is an effective public-safety program is key because the NYPD claims its benefits justify suspending one of our most basic constitutional protections: the right not to be searched by the police if not suspected of wrongdoing. This check on police power separates free societies from totalitarian ones, and for this reason the Supreme Court long has held that the police can search people without individualized suspicion in only the most extraordinary circumstances. Even then, suspicionless searches can be constitutional only if clearly effective in addressing a serious problem. Averting a terrorist attack of course is of paramount importance, but it is hard to see how a program of searching every tenth person at a small number of subway entrances and leaving those picked for search free to walk away and enter the system through another entrance can be effective in detecting or deterring the sophisticated terrorists this country now faces. The City nonetheless will try to convince a federal court, at a hearing scheduled for next week, that this program is so effective it warrants an unprecedented suspension of our constitutional rights. Like most New Yorkers, we rely on the subway and want it to be safe. And we fully recognize that concerns about terrorism weigh heavily in our society's ongoing effort to balance security and individual freedoms. We note that the NYCLU's challenge to the subway search program does not call into question any of the seemingly more effective options available to the Police Department, including a police presence at every subway entrance; increased intelligence gathering; use of use bomb-sniffing dogs; and stopping, questioning, and searching of people whose behavior is suspicious. What the police cannot and should not do, however, is deploy a system that results in thousands of innocent people being searched without any suspicion of wrongdoing in the absence of some compelling and immediate safety benefit. If the police are allowed to do this, we will have lost one of our most precious freedoms and will be opening the door to even greater threats to our way of life without having gained any meaningful safety. Dunn is associate legal director and Lieberman executive director of the New York Civil Liberties Union.