NEW YORK (CNN/Money) - If you were driving your sport utility vehicle to work today you might have heard on the radio that Dr. Jeffery Runge, the new head of the National Highway Traffic Safety Administration, has deemed your car "unsafe."

Related story • NHTSA: SUVs not safe enough

Your SUV's hefty weight and extra height might make it seem safer to you. In fact, SUVs have generally done well in the standard government crash-safety tests. Also, larger SUVs do have lower death rates per million registered vehicles than do smaller ones, so weight does seem to add to safety.

However, SUVs have a greater tendency to roll over in crashes, a tendency that may erase much of the benefit of the vehicles' added heft. Sport utility vehicles are designed to have high "ground clearance", the amount of space between the vehicle's underbody and the road. That gives them a greater tendency to tip over during sudden turns.

That tendency to roll over is the major reason that, when comparing vehicles of similar weight, SUVs have higher death rates than cars, said Russ Rader, a spokesperson for the Insurance Institute for Highway Safety, which analyzes government crash data. Passenger cars (a category that includes minivans) weighing 3,500 pounds had 106 deaths per million registered vehicles in the year 2000, while SUVs of that weight had 146.

In the real world, rollover crashes are relatively rare. But when they do happen, they are more likely to be deadly and they are much more likely to happen in an SUV.

In a speech Tuesday, Runge cited statistics that, while rollover accidents accounted for just 3 percent of all U.S. auto accidents in 2001, they caused nearly a third of all vehicle-occupant fatalities.

Single-vehicle crashes involving rollovers accounted for 41 occupant deaths per million registered passenger vehicles in 2000 compared with 9 deaths per million in multiple-vehicle crashes, according to the IIHS.

Single-vehicle rollover deaths have been far more common in SUVs than in cars. In 2000, the most recent year for which IIHS data are available, just over half of all occupant deaths in SUVs involved the SUV itself rolling over with no other vehicles involved. Such accidents accounted for just 19 percent of deaths in regular passenger cars.

In 2001, according to data from NHTSA, SUVs had the highest rollover rates of all vehicle types: 35.2 percent in fatal wrecks, 10.8 percent in crashes involving at least one injury.

Compounding the problems, according David Champion, director of automobile testing for Consumer Reports, is that SUVs' poor emergency handling characteristics leave them vulnerable to getting into roll-inducing situations to begin with. These situations can come as a surprise to drivers.

"It's very difficult for a driver to know how a vehicle is going to handle in an emergency situation because you just don't get into (emergencies) every day," he said.

Because of the designs of their bodies and suspension systems, SUVs are more likely to slide or skid under sudden maneuvers, Champion said. As vehicles slide, they often leave the road. It is there, when leaving the pavement, he said, that SUVs can get into real trouble as wheels catch in the soft ground and the vehicles tip and roll.

Simply wearing seat belts would help protect occupants in SUVs. In a September speech to the Association for Automotive Medicine, Runge pointed out that most rollover deaths in SUVs occur when people are ejected from the vehicle, something which is much less likely to occur with seatbelts buckled.

"This is exactly why GM has been diligently working with Dr. Runge and the NHTSA on increasing seat belt usage in this country," Jay Cooney, General Motors Corp.'s director of safety communications, said in a statement.

According to GM's analysis of NHTSA data "SUVs are among the safest vehicles on the road and have contributed to the dramatic decline in the nation's fatality rate over the last decade," he said.

NHTSA rates the "rollover resistance" of SUVs by comparing a vehicle's height to its width. The higher the vehicle is relative to width, the more likely it is to be to roll over, according to the agency. The ratings range from a top rating of five stars for SUVs like 2003 Acura MDX four-door four wheel drive to a low of 1 star for the 2003 2-wheel drive two-door Chevrolet Blazer.

In the real world, smaller SUVs, which are narrower and have shorter wheel-bases, are more prone to fatal rollovers than larger SUVs, according to the IIHS. In single vehicle crashes, fatality rates for light SUVs were six times higher than those for larger passenger cars. Champion, of Consumer Reports, said that those smaller SUVs also appeal to younger, less experienced drivers. This may be part of the reason for the higher death rate in those vehicles, he said.

Overall, Champion said SUVs are becoming safer as new technologies are brought to bear on the problem. One is stability control, which uses computers to help control the vehicle in emergency situations. Another is side-curtain air bags, which come down in the event of a rollover, covering the side windows protecting occupants' heads and upper bodies and helping prevent ejection from the vehicle.