Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges (2021) / Chapter Skim
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Pages 727-753
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727 APPENDIX F: ALGORITHM DESCRIPTION TABLE OF CONTENTS List of Figures ........................................................................................................................... 728 List of Tables .............................................................................................................................
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728 LIST OF FIGURES Figure 1. Main Subroutine .........................................................................................................
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729 ALGORITHM DESCRIPTION This document provides a description of the algorithms used in the predictive methods for evaluating freeways and ramps. These methods have been implemented in software in the Enhanced Interchange Safety Analysis Tool (ISATe)
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730 Evaluation Period The evaluation period is the set of years in the combined study period and crash period. Every calendar year in the evaluation period is separately evaluated using the methodology.
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731 Geometric Design • Number of through lanes. • Segment length.
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732 All total, there are 28 SPFs represented by unique combinations of the four conditions identified in the preceding list. Specific details about the SPF regression coefficients are provided in Section 18.6.1 of the proposed HSM Freeways chapter.
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733 Distribution percentages for these crash types are provided in Section 18.6.1 of the proposed HSM Freeways chapter. Application of these percentages is described in Sample Problem 1 in Section 18.13.1 of the proposed HSM Freeways chapter.
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734 Either of these two measures can be further disaggregated in terms of crash severity or crash type. For example, the "total estimated number of crashes for the study period" can be disaggregated into the following measures.
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735 Evaluation Type The predictive method can be used to evaluate one site, or a contiguous group of sites. The evaluation is described as one of several types, as determined by the analyst.
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736 • Lane width. • Inside shoulder width.
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737 Freeways chapter. The factor for ramp exit speed-change lanes is a constant (i.e., it is not a function of speed-change lane length)
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738 A procedure for calibrating the severity distribution functions is described in Section B.1.4 of the proposed HSM Appendix B for Part C Predictive Method The predictive method for freeway speed-change lanes is described as a flow chart in Figure 18-1 of the proposed HSM Freeways chapter.
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739 Evaluation Period The evaluation period is the set of years in the combined study period and crash period. Every calendar year in the evaluation period is separately evaluated using the methodology.
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740 Geometric Design • Number of through lanes. • Segment length.
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741 The overdispersion parameter is computed as a function of segment length. The equation for this calculation is provided in Section 19.6.1 of the proposed HSM Ramps chapter.
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742 A procedure for calibrating the severity distribution functions is described in Section B.1.4 of the proposed HSM Appendix A for Part C Predictive Method The predictive method for ramp segments is described as a flow chart in Figure 19-2 of the proposed HSM Ramps chapter.
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743 Evaluation Period The evaluation period is the set of years in the combined study period and crash period. Every calendar year in the evaluation period is separately evaluated using the methodology.
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744 • Ramp terminal configuration. Geometric Design Data for All Terminals • Number of through lanes on each crossroad approach.
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745 The units of measurement for all input data are U.S. customary.
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746 Specific details about the CMF formulation and regression coefficients are provided in Section 19.7.2 of the proposed HSM Ramps chapter. This section also identifies conditions where a CMF is not applicable.
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747 Output The output data computed using the predictive method consists primarily of the expected crash frequency for each site and year in the evaluation period. Useful performance measures and techniques for presenting this output are presented in the Output section in the part titled Algorithm Description for Freeway Segments.
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748 Compute CMFs (ComputeCMFs) Apply predictive model (ComputeNpredicted)
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749 The subroutine sequence in Figure 3 is shown to be repeated for each site. The first subroutine called in Figure 3a is used to calculate the CMFs for each year at the subject site.
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750 Compute CMFs (ComputeCMFs) Apply predictive model (ComputeNpredicted)
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751 Compute CMFs (ComputeCMFs) Apply predictive model (ComputeNpredicted)
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752 the associated facility component. The prefix is not shown in the table because the description offered is sufficiently general as to be applicable to all components.
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753 Table 2. Subroutines Implementing the Predictive Method Main Subroutine a Called Subroutine a Called Subroutine Description ComputePerformance ComputeMissingAADT Scans the AADT input cells for each year at each site.
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