All passenger vehicles sold in the United States must have shoulder and lap belts in the rear centre seat by the 2008 model year.

Three out of four new passenger cars already have the belts, but only half of pickups and sport utility vehicles do.

The U.S. National Highway Traffic Safety Administration's new rule requires 80 per cent of vehicles to have shoulder belts by the 2007 model year and 100 per cent the following year.

NHTSA estimates the rule will save up to 23 lives and prevent up to 495 injuries each year.

Auto makers support the rule, which will cost the industry $179-million (U.S. ) to $240-million, NHTSA said. It will cost up to $47 per vehicle to install the belts.

Shoulder belts have been required in rear window seats since 1989. The U.S. Congress directed NHTSA to require shoulder belts in middle seats as part of a 2002 law honouring Anton Skeen, a 4-year-old killed in a 1996 car crash in Washington state. Anton was wearing a shoulder belt but he was so small he slipped out of it.

Anton's death might have been prevented if he had been in a booster seat held in place by a shoulder belt.

Now that the new rule is in place, NHTSA Administrator Dr. Jeffrey Runge said the safest place for a child is in a booster seat in the rear centre, since that position offers the most protection in a side-impact crash.

Under the rule, auto makers can install either integrated seat belts, which are built into the seat, or detachable seat belts, which are often used in seats that can fold down or be removed.