The government yesterday warned consumers about the rollover risks posed by 15-passenger vans such as those used by church groups or airport shuttles, citing new research that shows fully loaded vans are five times more likely to flip than vans containing only a driver.

A separate study also found that electronic stability control systems are only moderately successful at preventing rollovers in such vans.

The big vans "are frequently misused, I think, by the general public because they're unaware of their limitations," said Jeffrey W. Runge, head of the National Highway Transportation Safety Administration, which issued the warning.

The agency issued similar warnings in 2001 and 2002, and Runge said the new alert was prompted by the studies and by the onset of summer, a time when churches and other groups are most likely to hit the road in large-sized vans. Commuter vanpools tend to have drivers who are experienced at handling the vans, Runge said, but "infrequent users who are at greatest risk need to hear this message."

The only 15-passenger vans currently made are two by General Motors -- the Chevrolet Express 3500 and the GMC Savana 3500 -- and the Ford E-350. There are some 500,000 such vehicles on U.S. roadways. Between 1990 and 2002, 1,576 15-passenger vans were involved in fatal crashes, resulting in 1,111 deaths to occupants, according to NHTSA.

Of those fatal crashes, 349 were single-vehicle accidents in which the van rolled over, NHTSA said.

NHTSA found that fully loaded 15-passenger vans are more likely to roll over than any other type of passenger vehicle, including cars, minivans, SUVs and pickup trucks. While electronic stability control improved handling in some driving tests, it did not in others, the agency found. And high speeds and curved roads greatly increased the chance that such a vehicle would flip.

Some safety advocates questioned why the agency simply repeated its warning rather than taking action against the vehicles. "What we need are better vehicles and not better warnings," said Clarence Ditlow of the Center for Auto Safety.

The fact that NHTSA found only limited benefit from electronic stability control, which uses sensors to adjust power and braking to keep the vehicle upright, suggests there is an inherent danger in the design of the tall vans, Ditlow said. "If these vehicles cannot be made safe they should not be used for passenger vehicles," he said.

Runge said there is not enough data to merit government regulation. "There may come a day when we have a performance requirement for these vehicles but we don't have one yet," he said. In the meantime, he said, the manufacturers are studying the situation.

"Until we have the data, we cannot begin to comment on the reports," a spokesman for Ford Motor Co. said. "We agree with NHTSA's recommendation that these vans be operated by experienced drivers who are familiar with the handling characteristics of their vehicles and that safety belts be worn by all passengers at all times."

General Motors Corp. said in a statement that its 15-passenger vans are equipped with safety features such as daytime running lights, four-wheel anti-lock brakes and air bags, and that its vans have an extra-long wheelbase that makes them more stable. The company also urged drivers to study their owner's manuals to better understand how "these and other full-sized vehicles may handle differently when fully loaded with passengers or cargo."

Runge said drivers have a great responsibility to handle the vehicles more safely, pointing out that more than three-quarters of the people killed in crashes of 15-passenger vans were not wearing safety belts.

He said he had firsthand knowledge of the potential for trouble with such vehicles. A few years ago on a ski trip, Runge rode in a shuttle van from the Denver airport to Aspen, the roof rack piled with luggage and skis. The driver lost control and the van wound up skidding sideways down the highway, he said. "Fortunately, we stayed on the pavement . . . and therefore we stayed upright."

But looking back, Runge said, he realizes he never should have gotten into such an overloaded, top-heavy vehicle. "Who knew?" he said.

NHTSA chief Jeffrey Runge has experienced safety problems in large vans firsthand.