PARIS – A vexing problem facing safety engineers working on a new-vehicle program is deciding the best position of the seatbelt buckle.

Low is best for safety, because a low belt on the lap prevents the occupant from sliding underneath, also known as submarining. But a high position is best for ergonomics.

TRW Automotive Holdings Corp. has developed what it calls an Active Buckle Lifter (ABL) that raises the buckle by 4 ins. (10 cm) when the door is opened. When the person buckles up, it descends to its safety position.

In a demonstration vehicle presented to auto makers and journalists here, a dynamic feature is added to the ABL. The buckle retracts a further 2.3 ins. (6 cm) when the car is braking in an emergency or during aggressive slalom driving. The device holds the occupant closer to his seat, safer in case of a collision and more comfortable in quick turns.

Automotive engineers are delighted with the ABL concept, says TRW engineer Martin Seyffert, but he admits the added mass of more than 2.2 lbs. (1 kg) poses questions for product developers.

However, because the system could replace one or two seatbelt pre-tensioners, he says the additional weight would be partially compensated.

An intelligent car could use the ABL as a haptic warning signal – sending little tugs to a drowsy driver, for example, which could be a valuable added safety feature.