Safety Performance of Part-Time Shoulder Use on Freeways, Volume 1: Informational Guide and Safety Evaluation Guidelines (2021) / Chapter Skim
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21 Chapter 1 – Crash Prediction Model Overview This chapter describes the crash prediction models (CPMs) developed in NCHRP Project 17-89 (Error!
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22 To facilitate interpretation and implementation of the CPMs developed for this project, the variable names and definitions used in these CPMs are consistent with those used in Chapter 18 of the HSM Supplement (2)
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23 boundaries are defined by the presence of a speed-change lane, a change in number of through lanes, or a relatively large change in the width of various cross section components. For the NCHRP Project 17-89 CPMs, segment boundaries are also defined by the start or end of a horizontal curve.
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24 Freeway segments (FI and PDO SPFs) : length of freeway segment, directional annual average daily traffic (AADT)
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25 Table 2. Adjustment factors in NCHRP Project 17-89 PDO crash prediction models.
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26 PTSU operation: no PTSU operation during any hour of the day PTSU lane width: 0 feet Turnout length in segment: 0.0 mile (i.e., not present) Ramp entrance speed-change lane length: 0.142 mile Ramp exit speed-change lane length: 0.071 mile The variable names and definitions used in the AFs are consistent with those used in Chapter 18 of the HSM Supplement (2)
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27 This AF is applicable to values of Pt,ptsu that range from 0.0 to 0.45. The number of through lanes range from 2 to 7.
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28 In Figure 2a, the site is shown as a freeway segment located upstream of a PTSU lane. The segment's length is shown to exceed that of the transition zone (which always equals 0.152 miles)
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29 The NCHRP Project 17-89 CPMs were developed exclusively using data from urban sites, and their applicability to rural sites is unknown. Traffic volumes were relatively high as many sites were on or near PTSU facilities that were implemented to reduce congestion.
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30 others in the future, but the proposed text presented in this document will serve in the interim as the definitive resource for those applying the model. 1.5 References 1.
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