The government today will propose a new safety standard for side-impact crashes that could result in more cars and trucks offering head-protection air bags.

The new standard could save as many as 1,000 lives a year by reducing the likelihood of head injuries during side-impact crashes, a spokesman for the National Highway Traffic Safety Administration said.

"It certainly has the potential for offering benefits that are far greater than any other regulation that's come out" since Jeffrey W. Runge took over as head of NHTSA in 2001, agency spokesman Rae Tyson said.

Tyson wouldn't discuss details of the proposed rule, saying only that it would set a goal for how vehicles should perform in a side-impact crash and describe the test automakers should use to measure compliance. It would then be up to each automaker to decide how to meet the standards, though head air bags are considered the best solution.

After the rule is proposed, there will be a comment period of about five months, and then the agency could issue a final rule that would be phased in, another spokesman said.

Improving side-impact safety has been one of Runge's top priorities at NHTSA. Such crashes were the second leading cause of vehicle fatalities last year, behind front-impact crashes, according to agency statistics. Overall, more people died last year on American roadways -- 43,220 -- than any time since 1990, though the rate of deaths per vehicle miles traveled was unchanged from the year before.

Side-impact crashes have become more deadly as the nation's fleet of vehicles has changed, with pickups and SUVs now more than half of all new vehicles sold each year. "How high those vehicles are off the ground makes them more aggressive in side impacts," said Adrian Lund of the Insurance Institute for Highway Safety, an organization funded by the insurance industry that tests vehicle safety.

When a passenger car is hit in the side by an SUV, the car's occupants are 22 times more likely to die than those in the SUV, according to NHTSA. When the car is hit by a large pickup, the car's occupants are 39 times more likely to die.

Concerned about such numbers, the Insurance Institute created a new side-impact test last year. Of 13 mid-size cars rammed by an SUV-shaped metal barrier, all but three received poor ratings, Lund said. The three that passed -- the Honda Accord, Toyota Camry and Chevrolet Malibu -- all had optional side head-protection air bags.

Fifty-four percent of 2004 cars and trucks offer such air bags either as standard equipment or as an option, said Eron Shosteck, spokesman for the Alliance of Automobile Manufacturers, a trade group representing most large automakers. Last year that group announced plans to take voluntary steps to improve side-impact safety, and Shosteck said yesterday that automakers expect NHTSA's proposed rule to be compatible with their own efforts.

"We don't know until we see the rule, obviously," he said. "But our goal and NHTSA's goal are the same, and that is to enhance side head protection."

NHTSA's current side-impact test, which also involves slamming a metal barrier into a vehicle, was designed in 1990, Tyson said. Since then, the agency has developed much more sensitive instrumentation for crash-test dummies and "became concerned because we were seeing some head injury scores that were certainly not good," Tyson said.