Safety Evaluation of Diverging Diamond Interchanges in Missouri

Title: Safety Evaluation of Diverging Diamond Interchanges in Missouri
Published: January 2015
Report number: cmr15-006
Project number: TR201406

Author(s): Dr. Praveen Edara, Dr. Carlos Sun, Boris R. Claros, and Henry Brown 

Performing organization: University of Missouri-Columbia, Department of Civil and Environmental Engineering 

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). 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 interchanges.