A series of controlled experimental programs were conducted for the purpose of improving the motor vehicle rollover protection system. Test results reported in this paper have been previously presented in SAE Paper No.980213 . Experiments tested lap belt restraints utilizing a variety of lap belt geometric and webbing slack conditions. Tests utilized in the series include dynamic and static tests and the use of test mannequins and human volunteers. In the first test program, utilizing a rigid seat, human volunteers were subjected to minus 1.0 Gz acceleration and a 95th percentile Hybrid III mannequin was subjected to minus 5.0 Gz acceleration for a variety of lap belt conditions. A second program utilized a rigid mannequin in production vehicle seats for the purpose of measuring and comparing seat belt system effective slack. Finally, the rigid mannequin from the second test and the rigid seat and lap belts from the first test were brought together and tested. The last test program provided information regarding factors affecting occupant displacement and flail in motor vehicle rollover crashes. Test methods and test device's were constructed to study and develop objective understandings of the effect of vehicle seats and seat belt systems for the purpose of improving or anticipating improvements to a motor vehicle rollover protection system. Overall, it is determined that occupant displacement from the seat in rollover conditions is affected by factors associated with a vehicle seat belt restraint system and seat. Compliance of live human occupant is shown to be a significant factor. Change in vehicle seat belt anchorage location and other reductions in effective slack of seat belt systems have the effect of substantially reducing occupant displacement. A variety of rollover injury mechanisms may be favorably influence by the demonstrated changes that reduce occupant rollover induced displacement.