Knee Airbag Supplemental Restraint Systems

Currently, 10 models of vehicles sold in the United States are equipped with driver’s knee airbag systems or with dual, driver and front passenger knee airbags. According to one U.S.-based airbag manufacturer, 3 million model year 2004 vehicles will be produced with “inflatable knee bolsters” (see it here).

Researchers report that hip injuries accounted for 48% of car-crash leg injuries in 1993-2001 vehicles (see “Knee Protection May Cause Worse Hip Injuries” by Jayne O’Donnell and James R. Healey, USA Today, May 18, 2003). To address this injury concern and to help keep front-seat occupants away from frontal airbag inflation zones, manufacturers have dramatically increased their use of knee airbag systems in 2004 model year vehicles.

Knee airbags are installed relatively low along the instrument panel, below the glove compartment on the passenger’s side of the dash. On the driver’s side, the knee airbag is positioned beneath the steering column. This relatively new technology works simultaneously with seatbelts and pre-tensioners to improve overall occupant survival in a frontal collision. In a frontal collision that initiates deployment of one or both frontal airbags, knee airbags deploy outward from the dash toward the front edge of the seat.

There are two basic knee airbag designs. One style uses the inflated knee airbag itself to press against the knees of the front occupants. The other knee airbag design has a “knee bolster” panel permanently attached to the bag itself. It is this padded panel that presses against the occupant’s knees, not the airbag.

This pressure against the knees and legs, along with the firing of the seatbelt pre-tensioners, holds the occupants in their proper seated position at the moment of a frontal collision. An inflated knee airbag increases the chance of the driver and front-seat occupant remaining out of the 10-inch and 18-inch inflation zones of the frontal airbags. It also reduces the risk of “submarining” – the gliding of occupants under their seatbelts, crushing them under the dash and instrument panel on the floorboard.

The popularity of knee airbag systems among automakers is also due in part to the fact that having knee airbags allows the “show surface” of the instrument panel to be designed farther from the front seats, giving the front seat occupant more legroom.