Intersection Crash Prediction Methods for the Highway Safety Manual (2021) / Chapter Skim
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Pages 197-209
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197 Chapter 9. Development of Models for Use in HSM Crash Prediction Methods: Crossroad Ramp Terminals at Tight Diamond Interchanges This section describes the development of crash prediction models for crossroad ramp terminals at tight diamond interchanges (TDs)
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198 Table 99. Site characteristic variables collected for crossroad ramp terminals at tight diamond interchanges Variable Definition Range of Permitted Values General Intersection Attributes Intersection configuration (i.e., number of legs and type of traffic control)
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199 Traffic data collection activities primarily involved accessing publicly available traffic volumes and statistics. Crash data were obtained from state DOTs.
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200 Figure 74. Example of a tight diamond interchange with the ramp boundaries definition All of the collected data (i.e., site characteristics, crashes, and traffic volumes)
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201 Traffic Volumes and Site Characteristics Traffic volumes and crash data from years 2011 through 2015 were used for analysis. Table 100 provides summary statistics for traffic volumes at the study sites used for model development.
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202 The findings with respect to some of these site characteristics are discussed in Section 9.3 on SPF development. Crash Counts All 51 interchanges included in the study experienced crashes.
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203 9.3 Safety Performance Functions -- Model Development SPFs for the crossroad ramp terminal of a tight diamond interchange were developed using Equation 57: 𝑁 = 𝑒𝑥𝑝 𝑎 + 𝑏 × ln(𝐴𝐴𝐷𝑇 )
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204 Before finalizing the models in Table 103, multiple models were developed testing other variables, such as traffic control type for right turns, number of left-turn lanes, number of rightturn lanes, number of channelized right turns, distance between terminals, number of driveways, and number of intersections. However, none of the parameters associated with the tested variables were statistically significant in the models.
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205 Figure 76. Graphical representation of the SPF for FI crashes at crossroad ramp terminals at tight diamond interchanges Figure 77.
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206 Following the development of the crash prediction models for crossroad ramp terminals at tight diamond interchanges, the research team conducted compatibility testing of the new models to confirm that the 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. The graphical representations of the crash prediction models in Figures 75-77 provide some sense of the reasonableness of the new models for crossroad ramp terminals at tight diamond interchanges.
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207 Figure 79. Comparison of crash prediction models for FI crashes at crossroad ramp terminals at tight diamond interchanges and single-point diamond interchanges Figure 80.
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208 Table 104 displays the distribution of crashes at tight diamond interchange crossroad ramp terminals by severity level. Table 105 displays the distribution of crashes at tight diamond interchange crossroad ramp terminals by collision type and manner of collision.
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209 9.5 Severity Distribution Functions Development of SDFs was explored for tight diamond interchange crossroad ramp terminals using methods outlined in Section 2.2.3 of this report. The database used to explore SDFs consisted of the same crashes and crossroad ramp terminals as the database used to estimate the SPFs but restructured so that the basic observation unit (i.e., database row)
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