Intersection Crash Prediction Methods for the Highway Safety Manual (2021) / Chapter Skim
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Pages 183-196
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From page 183... ...
183 Chapter 8. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Single-Point Diamond Interchanges This section describes the development of crash prediction models for crossroad ramp terminals at single-point diamond interchanges (SPs)
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184 Table 91. Site characteristic variables collected for crossroad ramp terminals at single-point diamond interchanges (Continued)
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185 Crash data were obtained from state DOTs. The crash data generally included details about the crash location (geographic coordinates)
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186 extends 100 ft beyond the outermost ramp connections, capturing crashes associated with the right-turn movements and the left-turn lanes. Figure 69.
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187 All of the collected data (i.e., site characteristics, crashes, and traffic volumes) were assembled into one database for the purposes of model development.
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188 Interchange geometric characteristics were collected using Google Earthยฎ and Google Street Viewยฎ (Table 91)
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189 Table 93. All crashes combined, single- and MV, and pedestrian and bicycle crash counts by crash severity -- single-point diamond interchange crossroad ramp terminals State Date Range Number of Sites Number of SiteYears Time of Day All Crashes Combined SV Crashes Multiple-Vehicle Crashes Pedestrian Crashes Bicycle Crashes Total FI PDO Total FI PDO Total FI PDO FI FI AZ 2011-2015 28 140 All 4071 1079 2992 287 83 204 3723 941 2782 18 43 UT 2011-2015 21 99 All 2133 504 1629 53 15 38 2040 454 1586 16 24 All states 20112015 49 239 All 6,204 1,583 4,621 340 98 242 5,763 1,395 4,368 34 67 Table 94.
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190 All SPFs were developed using a NB regression model based on all sites combined. Based on a review of the number of states, sites, site-years, and crashes for the database assembled, data for all sites were used for model development to maximize the sample size rather than using a portion of the data for model development and a portion for model validation.
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191 ๐ = ๐๐ฅ๐ ๐ + ๐ ร ln(๐ด๐ด๐ท๐ )
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192 Figure 72. Graphical representation of the SPF for FI crashes at crossroad ramp terminals at single-point diamond interchanges Figure 73.
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193 Table 96. SPF coefficients for crossroad ramp terminals at single-point diamond interchanges (based on Equation 57)
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194 new models provide reasonable results over a broad range of input conditions and that the new models integrate seamlessly with existing intersection crash prediction models in the first edition of the HSM was conducted. The graphical representations of the crash prediction models in Figures 71-73 provide some sense of the reasonableness of the new models for crossroad ramp terminals at single-point diamond interchanges.
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195 8.4 Crash Modification Factors During the development of the crash prediction models for crossroad ramp terminals at single-point diamond interchanges, three potential sources of CMFs for use with the SPFs were considered: โข CMFs developed as part of this research based on a cross-sectional study design and regression modeling โข CMFs already incorporated into the first edition of the HSM and applicable to crossroad ramp terminals at single-point diamond interchanges โข High-quality CMFs applicable to crossroad ramp terminals at single-point diamond interchanges developed using defensible study designs (e.g., observational before-after evaluation studies using SPFs -- the EB method) , as referenced in FHWA's CMF Clearinghouse with four or five-star quality ratings or based on a review of relevant intersection safety literature Based on a review of the CMFs already incorporated in the first edition of the HSM and other potential high-quality CMFs developed using defensible study designs, no CMFs were identified that were adaptable to the predictive models for crossroad ramp terminals at single-point diamond interchanges.
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196 8.6 Summary of Recommended Models for Incorporation in the HSM In summary, several crash prediction models were developed for crossroad ramp terminals at single-point diamond interchanges for consideration in the second edition of the HSM. The final models for FI and PDO severity levels presented in Table 96 are recommended for inclusion in the second edition of the HSM, consistent with existing methods in HSM Chapter 19.
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