Since 1995, the Insurance Institute for Highway Safety (IIHS) has evaluated the crashworthiness of more than 120 new vehicle models in a 64 km/h (40 mi/h), 40 percent offset deformable barrier crash test. The offset test is especially demanding of the vehicle structure, requiring only 40 percent of the vehicle width to manage the crash energy. Many of the models originally tested have been redesigned and retested, with the majority producing better structural performance than their predecessors. Critics of such testing have suggested that these tests are forcing vehicle stiffness too high for compatibility with other vehicles and other crash modes. IIHS has studied the relationship between vehicle mass, stiffness, and front-end length to the structural rating in the offset test. IIHS then studied vehicle accelerations, deformation, and interior intrusion for eight pairs of vehicles whose structural performance changed in the offset test after redesign for evidence of front-end stiffness changes. The data indicate that there were no significant correlations between mass, front-end length, and stiffness to structural performance in the offset test. The data also indicate that for models that sustained catastrophic collapse of the vehicle structure in the offset test, an increase in overall stiffness was required for better structural performance. The majority of vehicles whose structural performance improved did so without significant alteration to the stiffness of the vehicle for the first half-meter of deformation. These vehicles have maintained essentially the same stiffness in the front crush zone but have rapidly increasing stiffness as the deformation approaches the occupant compartment.