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35  Development of CMFs for Roadside Crashes on Rural Two-Lane Two-Way Roads 9.1 Background NCHRP 17Â72 was tasked with developing a procedure for estimating a CMF for roadside design. This chapter provides a brief overview of the development of the procedure for estimat ing the CMFs. Appendix N describes in detail the development of a procedure for estimating a CMF that describes the safety effect of roadside design elements associated with a rural two lane twoÂway highway. This CMF is intended to replace the roadside design CMF described in Chapter 10 of the HSM. Appendix O describes the procedural steps for computing the proposed roadside design CMF and using this CMF within the predictive model framework of HSM Chapter 10 to evaluate a road segment. The procedure was developed to quantify the effect of roadside design elements on the fre quency of singleÂvehicle runÂoffÂroad (SVROR) crashes. This effect is quantified in a manner that facilitates its use in the predictive model framework described in HSM Chapter 10. In the final step of the procedure, the CMF value is converted into a form that can be used with a model that predicts total crash frequency (i.e., all crash types and severity levels). 9.2 Development of a Roadside Design CMF for Rural Two-Lane Two-Way Roads Appendix N, âDevelopment of a Roadside Design CMF for Rural TwoÂLane TwoÂWay Roads,â includes three sections. The first section provides some background discussion on the roadside design CMF in HSM Chapter 10 and gives an overview of the model for predicting the frequency of SVROR crashes developed for NCHRP 17Â54, which was charged with developing one or more CMFs to replace the roadside design CMF. This section concludes with a discussion of some basic relationships between a CMF, the probability of a crash, and the odds of a crash. The second section describes the process used to develop and verify a proposed procedure for estimating the proposed roadside design CMF. Data reported in NCHRP 17Â54, the research literature, or both are used to individually examine the influence of each roadside feature on safety. The findings of the examination are used to derive equations for computing the probability of a crash associated with various roadside design elements. These probabilities are then com bined and used to compute the proposed roadside design CMF. The third section describes the development of a procedure for estimating the crash type distribution and the severity distribution for SVROR and nonÂSVROR crashes by K, A, B, C, and property damage only (PDO) levels. It extends the procedure for estimating the roadside design CMF value described in the second section. This section describes the crash type and severity distributions for a typical segment, the procedure for computing the crash distribution for a specific C H A P T E R 9
36 Crash Modification Factors in the Highway Safety Manual: A Review road segment of interest on the basis of the distribution of the types of crashes that occur on that segment, and the development of procedures for computing the predicted SVROR and nonÂSVROR severity distributions for the segment of interest. 9.3 HSM Implementation of a Proposed Roadside Design CMF for Rural Two-Lane Two-Way Roads Appendix O presents the procedure for computing the proposed roadside design in six sections. In each section, the changes to HSM Chapter 10 that are needed to implement the procedure for computing the proposed roadside design CMF are described. The first section describes the changes to the HSM predictive method. The second section describes the changes to the segmentation criteria. The third section describes the new roadside base conditions associated with the CMF. The fourth, fifth, and sixth sections describe the processes for computing the pro posed roadside design CMF, the frequency of specific crash types, and the frequency of specific crash severity levels, respectively. The last section illustrates the use of the proposed procedure to evaluate a road segment when information is available to compute the proposed roadside design CMF.
