Automobile Seat Belts: Usage Patterns in Automatic Belt Systems

The use of manual (i.e., active) restraint systems (or seat belts) in cars has increased dramatically over the last several decades, changing from around 10% in the late 1970s to as much as 68% in the 1990s (Datta & Guzek, 1992; National Highway Traffic Safety Administration [NHTSA], 1996b). Several factors are responsible for this increase. One important determinant has been the passage of mandatory use laws (MULs), which have been shown to increase use rates by roughly 2.5 times when applied and enforced vigorously (Bunch, Hatfield, Hinshaw, & Womack, 1986; Reinfurt, Campbell, Stewart, & Stutts, 1988, 1990; Ulmer, Preusser, Preusser, & Cosgrove, 1995). Another factor has been the development of so-called automatic seat belt systems that satisfy Federal Motor Vehicle Safety Standard 208. This law requires that all vehicles sold after 1991 in the United States must have passive occupant restraint systems – either automatic shoulder belts or airbags – for the front outboard passengers. To meet this standard with belt systems, manufacturers have designed a surprising array of two-, three-, and four-point restraint systems, for which the number of points refers to the number of attachments of the belt to the vehicle (see Padmanaban, Lange, Ray, Curzon, & Cooperrider, 1991, for a description). In one of these systems (the subject of this paper), which has been adopted by several automakers, an automatic (motorized or nonmotorized) shoulder belt is combined with a manual lap belt. Systems with automatic shoulder belts have been responsible for a dramatic increase in the use of shoulder belts; estimates of use rates have increased from roughly 45% to 50% for manual shoulder belts to 97% for the nondetachable motorized shoulder belts (Reinfurt, St. Cyr, & Hunter, 1990; Streff & Molnar, 1991). Recently, however, there has been concern that these seat belt designs might actually decrease the tendency for drivers and frontseat occupants to use the manual lap belt Automobile Seat Belts: Usage Patterns in Automatic Belt Systems