Vehicle Automation and Transportability of Crash Modification Factors

Although the Highway Safety Manual (HSM) now provides empirical tools for predicting the safety consequences of highway engineering decisions, these tools represent the prevailing driver and vehicle conditions in the United States during the last few decades. As automated vehicles improve in capability and increase in market share, these conditions will change, possibly affecting the accuracy of HSM predictions. This report investigates the feasibility of using "transportability" analyses, developed by Judea Pearl and Elias Bareinboim, to assess the "transferability" of crash modification factors (CMF) to new situations. An overview in Chapter 2 concludes that transportability analysis is, in principle, possible provided one can describe a causal mechanism that explains how a CMF works. Chapter 3 then describes developing such an explanation for pedestrian hybrid beacons (PHB). In Chapter 4 the explanatory model developed in Chapter 3 is used to assess the transportability of existing estimates of PHB CMFs to a hypothetical situation where vehicles with autonomous braking are present.