A method was developed for ranking intrastate carriers in Wisconsin to enhance the selection of carriers for Wisconsin's intrastate compliance (comprehensive safety) review program. The federal SafeStat methodology for identifying interstate motor carriers that have poor safety records cannot be used in Wisconsin because of the lack of carrier size data. Missouri's intrastate SafeStat methodology, which does not require carrier size data, was used as the starting point in developing Wisconsin's methodology. The methodology involves merging a ranking of carriers based on historical inspection data (the total out-of-service rate) with a ranking based on total crashes. The validation based on crashes per carrier for a subsequent period showed a similar pattern for the before and after crash rates by percentile group. The overall level of crashes for the after period, however, was substantially lower because of the statistical phenomenon of regression to the mean (RTM). The methodology is also limited by the lack of exposure data for the carrier crash data. To account for RTM, an empirical Bayesian method for estimating RTM was proposed and applied to a before-and-after analysis of intrastate carrier crashes in Wisconsin. The method provided estimates of the after-period crash rates that were much better than the naive assumption of no change in the crash rate. An improved fit for the RTM estimates is likely if more homogeneous groups of carriers were created on the basis of grouping by carrier size.
Abstract