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Safety evaluation of diverging diamond interchanges in Missouri.
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  • Abstract:
    The Diverging Diamond Interchange (DDI) has gained in popularity in the United States during the last decade. The

    operational benefits and lower costs of retrofitting a conventional diamond with a DDI have contributed to its increased use.

    Existing research on DDIs has focused primarily on the assessment of operational benefits. Unfortunately, formal safety

    evaluations of DDIs are lacking. This study filled the knowledge gap by conducting a safety evaluation at the project-level

    (interchange) and the site-specific level (ramp terminals) of DDIs using three types of before-after evaluation methods: Naïve,

    Empirical Bayes (EB), and Comparison Group (CG). Three evaluation methods were used since the methods involved

    different trade-offs, such as data requirements, complexity, and regression-to-the-mean. The safety evaluation at the project-level accounts for the influence of the DDI treatment in the entire footprint of the interchange. On the other hand, the site-specific approach focused on the influence at the ramp terminals only. All three methods showed that a DDI replacing a

    conventional diamond decreased crash frequency for all severities. At the project-level, the highest crash reduction was

    observed for fatal and injury (FI) crashes – 63.2% (Naïve), 62.6% (EB), and 60.6% (CG). Property damage only crashes were

    reduced by 33.9% (Naïve), 35.1% (EB), and 49.0% (CG). Total crash frequency also decreased by 41.7% (Naïve), 40.8%

    (EB), and 52.9% (CG). Similarly, in the site-specific analysis, the highest crash reduction was observed for fatal and injury

    (FI) crashes – 64.3% (Naïve), 67.8% (EB), and 67.7% (CG). Property damage only crashes were reduced by 35.6% (Naïve),

    53.4% (EB), and 47.0% (CG). Total crash frequency also decreased by 43.2% (Naïve), 56.6% (EB), and 53.3% (CG). A

    collision type analysis revealed that the DDI, as compared to a diamond, traded high severity for lower severity crashes. While

    34.3% of ramp terminal-related FI crashes in a diamond occurred due to the left turn angle crashes with oncoming traffic, the

    DDI eliminated this crash type. In summary, the DDI offers significant crash reduction benefits over conventional diamond


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