The empirical Bayes (EB) methodology is now generally accepted as the state of the art in conducting observational before-after studies of the safety effect of measures applied to roadway sites. There is, therefore, a natural tendency to put a stamp of approval on any study that uses this methodology, and to assume that the results can then be used in specifying crash modification factors for use in developing treatments for hazardous locations. At the other extreme are skeptics who suggest that the increased sophistication and data needs of the EB methodology are not worth the effort since alternative, less complex methods can produce equally valid results. The objectives of this paper are twofold. The first is to provide evidence from actual studies that the EB methodology, if properly undertaken, does produce results that are substantially different and more valid than those from more traditional types of studies. The second objective is to emphasize that caution is needed in assessing the validity of studies undertaken with the EB methodology and in using these results for providing crash modification factors. To this end, a number of issues that are critical to the proper conduct of EB evaluations are raised and illustrated based on recent experience with EB evaluations. These include: amalgamating the effects on different crash types; the specification of the reference/comparison groups; and accounting for traffic volume changes (A). For the covering abstract of the conference see ITRD E217780.
Abstract