Independent review: statistical analyses of relationship between vehicle curb weight, track width, wheelbase and fatality rates

Abstract

NHTSA selected the vehicle footprint (the measure of a vehicle’s wheelbase multiplied by its average track width) as the attribute upon which to base the CAFE standards for model year 2012-2016 passenger cars and light trucks. These standards are likely to result in weight reductions in new light duty vehicles. As part of its regulatory analysis, the government would like to estimate the effect of the new CAFE standards on safety in terms of crash injuries and fatalities. A number of fairly comprehensive statistical papers have been published analyzing associations between fatality/injury rates and vehicle weight, track width, and wheelbase. Many of the papers arrive at conclusions that are inconsistent. This report is a review of papers analyzing associations between crash/fatality outcome and vehicle weight and size. The various studies are based on different data sources, model assumptions, and methodologies. The authors of these studies represent a mix of those in government, research institutes, and academia, and have a broad range of professional backgrounds and philosophies. The goal of this report is to provide an independent review of the papers and to critically assess the methods and conclusions presented. The review is independent in the sense that it was conducted by a third party without any interest in the reported outcome. This review focuses on issues such as multicollinearity, data sources, the use of logistic regression, and induced exposure methods. Comments and suggestions are also made with regard to methods used in the various papers.