Federal regulators are warning consumers about the rollover risks posed by 15-passenger vans, including those used by church groups and airport shuttles. The National Highway Transportation Safety Administration (NHTSA) said new research shows that fully loaded vans are five times more likely to flip than vans containing only a driver.

In fact, 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.

Another study found that electronic stability control systems are only moderately successful at preventing rollovers in the big vans. A third study found that in crashes between 1990 and 2002, three-quarters of the 684 people killed in single-vehicle van crashes weren't wearing a seat belt.

Between 1990 and 2002, 1,576 15-passenger vans were involved in fatal crashes, resulting in 1,111 deaths to occupants, NHTSA said.

NHTSA head Jeffrey Runge said the vans are "frequently misused" by consumers who are unaware of their limitations. The vans are banned for use by schools but are frequently used by youth groups, senior centers, colleges and other organizations likely to rely on untrained, inexperienced drivers.

Among NHTSA's safety recommendations:

• Use only trained, experienced drivers.
• Insist that all occupants wear safety belts at all times. An unrestrained 15-passenger van occupant involved in a single vehicle crash is about three times as likely to be killed as a restrained occupant.
• If possible, have passengers and cargo forward of the rear axle and avoid placing any loads on the roof.
• Check your tires: Excessively worn or improperly inflated tires can lead to a loss-of-control situation and a rollover. At least once a month, check that the van's tires are properly inflated and the tread is not worn down.

NHTSA warned drivers to be particularly careful when driving the vans at more than 50 m.p.h, or around curves. NHTSA issued similar warnings in 2001 and 2002. It's being repeated because of increased use of the vans during the summer.

Safety advocates were unimpressed by the warnings.

"What we need are better vehicles and not better warnings," said Clarence Ditlow of the Center for Auto Safety, according to The Washington Post. "If these vehicles cannot be made safe they should not be used for passenger vehicles."

Runge said manufacturers are "studying the situation."

General Motors said its 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. Drivers should study their owner's manual, GM said.

GM and Ford are the only manufacturers currently making 15-passenger vans -- the Chevrolet Express 3500, GMC Savana 3500 and the Ford E-350. There are some 500,000 such vehicles on U.S. roadways.

The study was released two days after a fiery van crash that killed three members of a New York church group near Niagara Falls. Two of the three weren't wearing seat belts, Ontario police said. The van was carrying 12 people when it crashed.

The complete text of the three studies cited by NHTSA can be found on the agency's Web site.