Cars are doing a better job of protecting occupants in crashes with sport-utility vehicles and pickup trucks, even as light trucks have gotten bigger and heavier, according to a new study by the Insurance Institute for Highway Safety.

In cars that collided with midweight SUVs, the death rate fell 39 percent -- to 42 deaths for each one million registered SUVs from 2000 to 2003, compared with 69 deaths for each one million registered SUVs a decade earlier. The decline in the death rate was more pronounced in collisions involving heavier SUVs -- to 49 deaths for each one million registered SUVs from 86.

One of the biggest issues raised by car-safety advocates in recent years has been the disproportionate harm that SUVs can do to the passengers of the cars that they hit. While statistics show that in collisions between the two types of vehicles, the occupants of cars still are much more vulnerable than are those in SUVs, the study suggests the application of new technology and greater use of seat belts may help to mitigate this gap.

The Institute, the Arlington, Va., research arm of auto insurers, specifically looked at the differences in height and weight of vehicles and how that mismatch affects death rates in crashes. It found that while incompatibility still is an issue, it is less of a problem than it was a decade ago.

Incompatibility -- the idea that people in cars are more at risk in collisions with SUVs and trucks because those vehicles are heavier -- has been a focus of recent efforts to make vehicles safer. That includes the million-plus dollars the Institute has put into a crash-test program that mimics an SUV slamming into the sides of other vehicles. It is also one of the federal government's top auto-safety priorities.

Auto makers also have agreed to make voluntary changes to vehicles to address the mismatch issue. Results of those commitments -- which include adding head-protecting air bags and realigning vehicle frames on SUVs and trucks -- wouldn't show up yet in accident statistics, though.

Instead, the Institute points to more basic safety measures to explain the improvement during the past decade: better vehicle designs, increased use of seat belts and the fact that half of all registered cars now are equipped with driver air bags, compared with 3 percent in 1990. SUVs also have changed over that time period, going from heavier, more aggressive truck underbodies to SUVs built on car frames that inflict less harm in a crash.

Concern about the dangers light trucks pose to cars has increased as SUVs have proliferated on the roads. Last year, SUVs and light trucks accounted for 55 percent of new-car sales, up from 40 percent a decade ago.

While cars have gotten safer in crashes with SUVs, the safety of the SUVs has changed little, if at all. In crashes with cars, the death rates for people in both SUVs and pickup trucks generally improved only slightly in most weight categories during the same time period. The death rates were slightly worse for the lightest and heaviest SUVs.

Still, the death rates for occupants of SUVs is substantially lower than it is for car occupants when the two vehicles collide, and as the SUVs get heavier, they get safer for their own occupants but deadlier for the cars involved in the crash. (The study looked only at two-vehicle crashes, so the increased rollover risk of SUVs, which is mostly an issue with single-vehicle accidents, wouldn't be included.)

Death rates were 59 percent higher for car occupants than for SUV occupants in crashes involving the lightest SUVS. In the heaviest SUVs, the death rates for car occupants were nine times as high as those of SUV occupants.

The study looked at deaths in 2000 to 2003 in model-year 1999 to 2002 vehicles compared with 1990 to 1993 deaths in 1989-1992 model-year vehicles.

Institute President Brian O'Neill said that he is encouraged by the improvements and expects that changes from the voluntary agreements, which still need to be phased in, will have a significant impact going forward. When the improvements make their way into the fleet, Mr. O'Neill said fatality rates in side-impact crashes could decline 30 percent to 40 percent.

Two years ago the auto industry, at the prodding of the Insurance Institute and to ward off potential government regulation, agreed to address the mismatch issue. As a result, all new vehicles will get head protection by the end of the decade, while SUVs and pickups will get design changes to their front ends to prevent them from riding over cars. All the major auto makers have agreed to the changes; the improvements are being made in stages.

The Alliance of Automobile Manufacturers, which represents car makers, said the companies are on track to meet their first of deadlines.

Auto makers said increased use of seat belts also makes a difference. Auto safety "is a shared responsibility between us and the driving public. Until we get everybody buckling up on every trip, we will still see far too many deaths and injuries on the nation's roads," says Bob Lange, safety chief at General Motors Corp.

Currently, about 80 percent of drivers use seat belts, up from 50 percent in 1990.

Road Safety

Some of the latest statistics on SUVs:

The death rate for occupants of cars that collided with heavy SUVs fell 43 percent, according to a new study.

Death rates are 59 percent higher for car occupants than SUV occupants in crashes involving the lightest SUVs.

SUVs account for 20 percent of newer registered vehicles, up from 7 percent a decade earlier.

-- Some 80 percent of drivers now use seat belts, compared to 50 percent in 1990.