Two insurers that together cover nearly 25 million vehicles in the US have quietly begun making drivers of bigger vehicles pay more for liability insurance, the New York Times (NYT) reports.

Allstate Insurance Company and the Progressive Insurance Group, the nation's second- and fourth-largest insurers, confirmed to the newspaper that they had begun raising the cost of liability insurance for many big, high-riding vehicles while lowering premiums for the cars that are owned by most Americans.

The NYT also said that the Farmers Insurance Group, the third-biggest insurer, plans to adopt similar pricing next year.

Insurers until recently have hesitated to adopt pricing plans that shift costs to owners of the biggest vehicles.

State Farm, the nation's largest auto insurer, recently announced a new pricing plan that would do just the opposite, reducing the cost to drivers of many larger vehicles for the personal injury portion of their coverage.

The NYT says that the otherwise contradictory moves have in common that they reflect aspects, at least, of reality.

When big, high-riding vehicles collide with smaller ones, the smaller car is often left severely damaged and its occupants with severe injuries, while the larger vehicle better protects its occupants. Yet, until now, insurance premiums have ignored that pattern.

Accident data are now persuading the industry to make changes that, officials say, better assign costs to the responsible parties. For consumers, premiums are likely to rise or fall by $US150 or less a year, insurance executives told the NYT.

State Farm, which insures 37 million cars, is holding out against raising liability rates for drivers of bigger vehicles, arguing that while they indeed cause more damage in crashes, they are involved in fewer accidents overall. Allstate says the greater damage caused by bigger vehicles more than offsets the smaller number of crashes.

Owners of minivans and of large and mid-size cars will be able to cut their insurance costs the most under all the companies' pricing plans. These vehicles generally provide excellent protection for their occupants, the industry has found, yet their ample "crumple zones," which absorb the force of crashes, also limit the harm to other motorists.

But the NYT says that the picture will be different for drivers of sport utilities, which have few crumple zones.

Consumer advocates applauded efforts to assign costs more accurately. "We've been saying they should do this for some time," the NYT quoted J. Robert Hunter, the director of insurance for the Consumer Federation of America, as saying. "Now the people doing the most damage will be paying the most."

The NYT says that today's large sport utilities are also far deadlier to other motorists than large cars, because the SUVs have stiff, high underbodies that ride up over the bumpers and door sills of cars during collisions, slamming into the passenger compartment.

US federal regulators calculate that midsize sport utilities like the Chevrolet Blazer and Nissan Pathfinder are three times as likely to kill other motorists in a crash as large cars of the same weight.

Having become aware of the design problem three years ago, the major automakers are redesigning their SUVs. More than half the midsize and full-size sport utilities sold next year will have been redesigned, the NYT said.

Without announcing the shift, Allstate, which insures about 20 million vehicles, began imposing the new pricing structure in Oregon late last year. It is now applying the new rates to 2000 model-year vehicles there, as well as in Florida, Illinois and Louisiana, and has won approval from 13 states, including New York, to apply the rates on 2001 model-year vehicles. Progressive also made no announcement about its pricing method.