Safety Performance of Part-Time Shoulder Use on Freeways, Volume 2: Conduct of Research Report (2021) / Chapter Skim
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17 C H A P T E R 3 - M O D E L I N G A P P R O A C H E S Modeling Approaches This chapter describes the site types studied in NCHRP Project 17-89 and the framework for the crash prediction model (CPM) associated with each site type.
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18 Figure 2. Illustrative sites for one-directional freeway facility evaluation.
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19 Each predictive model equation includes a safety performance function (SPF) , one or more adjustment factors (AFs)
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20 The second option is to develop one CPM for predicting FI crash frequency and a second CPM for predicting total crash frequency. With this option, PDO crash frequency is computed by subtracting the predicted FI crash frequency from the predicted total crash frequency.
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21 Equation 3 π , , , πΆ , , π , , , π΄πΉ , , , β¦ π΄πΉ , , , Equation 4 π , , , πΆ , , π , , , π΄πΉ , , , β¦ π΄πΉ , , , where Np,fs,at,z = predicted average crash frequency of a freeway segment for all crash types at and severity z (z = fi: fatal-and-injury, pdo: property-damage-only, as: all severities) (crashes/year)
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22 for inclusion in a future edition of the HSM. The remainder of this section presents an overview of the three model types and explains the differences between them.
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23 Adapt the Existing HSM Part C Freeway CPM This approach involved adapting the existing HSM freeway CPM such that it could be used to evaluate freeways with PTSU operation. The adaptation process would have led to the development of new PTSUrelated AFs that would be used with the existing HSM method to evaluate freeways with PTSU operation.
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