Abstract
A site-specific analysis of the safety of diverging diamond interchange (DDI) ramp terminals was conducted for the first time. Crash modification factors (CMFs) were developed for DDI ramp terminals for different crash severity levels. The ramp terminal CMFs complement the project-level DDI CMFs developed in previous research. The study used data from 20 ramp terminals in Missouri. The safety evaluation was conducted with two before-and-after observational methods: comparison group (CG) and empirical Bayes (EB). Because of inaccurate crash locations, a systematic crash location correction process was developed. An extensive review of 8,400 individual crash reports was conducted for the calibration and safety evaluation. Both before-and-after safety evaluation methods produced consistent results. The DDI design replacing a conventional diamond decreased the numbers of ramp terminal–related crashes for all severity levels. The most significant crash reduction by method was observed for fatal and injury (FI) crashes: 73.3% (CG) and 63.4% (EB). Property damage only (PDO) crashes were reduced by 21.0% (CG) and 51.2% (EB). The total crash frequency also decreased by 42.7% (CG) and 54.0% (EB). The EB CMF values for the DDI ramp terminal were 0.366 for FI crashes, 0.488 for PDO crashes, and 0.460 for total crashes. This study serves as a case study for conducting a site-specific analysis of ramp terminals or other interchange facilities. The methodology that was used is transferable and can be used to quantify the safety effects of other innovative intersection and interchange designs.
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