It is worth noting that N.H.T.S.A. uses the term “ejection mitigation,” an acknowledgment that there is no practical way to provide total protection. In the vernacular of safety, there are few absolutes.

In contrast to earlier safety standards, the government is not leaving it up to automakers to determine which technology is used to meet the standard. Rather, it is requiring carmakers to develop side-curtain air bags that, when deployed, will cover the full opening of each side window adjacent to the first three rows of seats and a portion of any windows in a cargo area.

Steve Cassatta, a senior staff engineer at General Motors, said in a telephone interview that rollover-capable side-curtain air bags were part of G.M.’s strategy before the new rules went into effect. “From a technology standpoint, it isn’t inconsistent with our approach,” he said.

Current side-curtain air bags, engineered to be effective with seat belts, deploy from above the windows and help protect occupants in side impacts and rollovers. But they do not provide enough window coverage to assure that unbelted passengers will be kept inside the vehicle.

Jim Vondale, who until recently was the director for automotive safety for Ford Motor — the company that introduced side-curtain air bags in 2002 — said in a telephone interview that to meet the ejection test requirements, the curtain air bags would have to be considerably larger than those in today’s vehicles. He said the air bags would be tucked behind the headliner and roof pillar trim, possibly intruding on interior space somewhat.

Safety standards mandated by the government have generally been engineered with the presumption that safety belts were being used. Some experts see a risk that the new regulation, by mandating protection for unbelted occupants, could undo the decades of progress that have resulted from campaigns that promoted belt use.

“One of the concerns we have about having to design systems that protect unbelted people is it sends an inconsistent message,” Mr. Vondale said. “We want to consistently say that the most important element is the safety belt.”