The National Highway Traffic Safety Administration is trying to determine if it should force auto makers to do what most of them are already doing or planning to do voluntarily -- install rear seat belts with shoulder harnesses in new cars.

In a notice published yesterday in the Federal Register, NHTSA said it is looking for "comments on whether the agency should require manufacturers to install lap/shoulder belts in rear seating positions of passenger cars, multipurpose vehicles and small buses. . . . The agency is specifically seeking comment on the cost-effectiveness of requiring rear seat lap/shoulder belts, by vehicle type."

Auto safety advocates favor such a rule, noting that both federal and private studies conclude that seat belts with shoulder harnesses offer better protection in crashes than single lap belts that fit across the waist.

However, since most major auto makers are installing the shoulder-harness belts in front and rear vehicle seats, safety advocates said they did not understand why NHTSA did not proceed directly with rulemaking rather than going through this preliminary comment stage.

"NHTSA is just playing a silly bureaucratic game," said Brian O'Neill, president of the Washington-based Insurance Institute for Highway Safety, a research and lobbying group for most of the nation's major auto insurers.

"It's beyond me why we're going through this" information-gathering step, O'Neill said. "I don't think that there's any argument anywhere about the superiority of shoulder-harness belts over lap belts."

NHTSA officials yesterday agreed that their procedure "is unusual in that it would specify regulations even though many auto manufacturers already are installing lap/shoulder belts in the rear seats of their vehicles, or have made commitments to do so."

NHTSA is simply being cautious, seeking "to make sure that it doesn't make a ruling that would interrupt the progress" that is being made voluntarily, an agency spokesman said.

That explanation didn't satisfy the critics, including the National Transportation Safety Board, a companion federal transportation agency.

"We were hoping that NHTSA would have just gone directly to rulemaking," said Alan Pollock, spokesman for NTSB. Pollock's agency last July issued a report saying that rear-seat, lap-only belts could cause serious internal injuries to car occupants in crashes.