The number of passenger vehicles with combined 3-point belt/driver air bag restraint systems is steadily increasing. To investigate the effectiveness of this restraint combination, 48 kph frontal collisions were performed with human cadavers. Each cadaver's thorax was instrumented with a 12-accelerometer array and two chest bands. The results show, that by using a combined standard 3-point belt (6% elongation)/driver air bag, the thoracic injury pattern remained located under the shoulder belt. The same observation was found when belts with 16% elongation were used in combination with the driver air bag. Chest contours derived from the chest bands showed high local compression and deformation of the chest along the shoulder belt path, and suggest the mechanism for the thoracic injuries. On the other hand, in tests where the air bag was the only available torso restraint, forces were distributed uniformly over the front of the chest;​ high local compression/deformation and injuries were reduced.

This study asks if it is possible to obtain both the thoracic injury mitigating benefits of an air bag only restraint and the all-impact-direction benefits of the belt from a combination restraint system by adding a force limiter to the shoulder belt. For this reason, tests with force limiters were performed. Initially, the investigation was carried out with Hybrid III dummies using two different levels of force limiters:​ 4 kN and 5 kN. The force limiter with the level of 4 kN showed, through examination of the chest band contours, a more bag-like uniform compression of the chest, with the belt effect only slightly pronounced. The chest compressions were 4 to 8 cm, and the resultant spinal accelerations were 30 to 40 g's.

By using the same restraint combination and force limiter, comparable vertebral accelerations and chest compressions were measured in cadaver testing. No injuries were found in the cervical spine and only an AIS 1 was observed in the thorax, for the age range of 60 to 65 years. The results also suggest that when a driver air bag is combined with a 3-point belt system that limits the torso belt loop load to 4 kN, additional injury mitigation benefits for both the cervical spine and the thorax are obtained in frontal collisions.

Analytical simulations were also conducted using different size occupants in both the baseline and the optimized belt/air bag restraint and in other crash conditions. These simulations suggest the harmonized belt/air bag also improves safety performance for other than mid-sized male occupants and does not adversely affect the performance of the restraint system in other prevalent crash configurations.