In a previous NHTSA research program, the Door Effective Padding Thickness (DEPTH) was found to be highly correlated with side impact occupant protection. Using an expanded set of twenty-eight production car side impact tests and a series of modified vehicles side impact tests, this paper reexamines the question of how vehicle design influences side impact thoracic injury. The findings of this extended study confirm the conclusion of the earlier paper: Of the design variables investigated, DEPTH is the single design variable most strongly correlated with thoracic side impact injury potential.