ASHINGTON, Nov. 5 — Police roadblocks, and the circumstances under which the police can impede the free movement of drivers whom they have no reason to suspect of a crime, are familiar territory for the Supreme Court. The court has upheld roadblocks to catch drunken drivers and, near the country's borders, illegal immigrants. It has struck down those aimed at finding illegal drugs.

The justices wrestled in an argument on Wednesday with still another variant: "informational checkpoints" designed to find not wrongdoers but witnesses to a crime, in this case a fatal hit-and-run accident in a Chicago suburb. A week after the accident, at the same location and late hour, the police in Lombard, Ill., blocked the road and handed each driver a flier that described the incident and asked for any information about it. One approaching driver nearly hit one of the officers with his car, leading to a sobriety test and a drunken driving conviction.

The question for the justices was whether the roadblock was a valid investigatory tool or, under the Fourth Amendment, an unreasonable and therefore unconstitutional seizure. The Illinois Supreme Court, dividing 4-to-3, found the roadblock unconstitutional and overturned the conviction of the driver, Robert S. Lidster. The state appealed.

Whether the justices turn out to be as closely divided as their Illinois counterparts or not, it was clear on Wednesday that they found the case intriguing and challenging. On the one hand, "why isn't this the most reasonable thing in the world?" Justice Stephen G. Breyer asked Donald J. Ramsell, Mr. Lidster's lawyer. "This is not much of a demand, to stop for 10 seconds to find someone who killed someone," the justice continued. "Why is that unreasonable?"

On the other hand, "there should be some limiting principle," Justice Anthony M. Kennedy told Gary Feinerman, the Illinois solicitor general. Justice Kennedy's comment reflected several justices' concern that these checkpoints, once endorsed by the Supreme Court, could become routine.

Justice Antonin Scalia envisioned the police stopping every car to ask, "Sir, or Madam, have you seen any crime committed in the last six months?" He asked Mr. Feinerman if that be would be "good or bad."

"In most instances, bad," Mr. Feinerman replied. But he added that it might be acceptable in a city overcome by "rampant lawlessness," with hundreds of unsolved crimes and people afraid to approach the police openly, for officers to stop drivers and distribute fliers with numbers that witnesses could call privately.

But crimes occur every day in major metropolitan areas, Justice Ruth Bader Ginsburg objected. Police chiefs would inevitably have a range of views on when and whether to use the technique, she said, depicting a roadblock-strewn landscape lacking in standards or consistency.

Mr. Feinerman urged the court not to rule out the roadblocks completely. Each instance should be evaluated by balancing the intrusion against the law enforcement justification, he said.

The Justice Department entered the case on the state's side. Patricia A. Millett, an assistant solicitor general, said that an informational checkpoint differed significantly from the narcotics-search checkpoint that the court found unconstitutional in a decision three years ago, City of Indianapolis v. Edmond.

Ms. Millett said that while the roadblock in the earlier case involved "seizing people first and looking for a crime," the informational checkpoint was a sensible aide to solving a crime that was known to have taken place.

But Mr. Ramsell, the lawyer for Mr. Lidster, argued that there was little difference between the two cases. Like the earlier decision, he said, his client's case involved the "mass suspicionless seizure of ordinary citizens for the purpose of investigating criminal wrongdoing."

In this case, Illinois v. Lidster, No. 02-1060, the police set up the roadblock exactly a week after the accident on the theory that people might be in the same place at the same time. In fact, while the roadblock did not find a witness, local television coverage of it prompted a viewer to call the police and identify a suspect.

This sequence of events led some justices to wonder whether the police could skip the roadblock and simply make a televised appeal for witnesses to come forward. But "maybe television will only cover roadblocks," Chief Justice William H. Rehnquist commented.

Justice David H. Souter then observed that "if they had someone like Justice Scalia to go on the screen," there would be ample coverage. Justice Scalia is notoriously camera-shy. There was much knowing laughter in the courtroom.