Safety Evaluation of Diverging Diamond Interchanges in Missouri

The diverging diamond interchange (DDI) has gained in popularity since its first implementation in the United States in 2009. The operational benefits and lower costs of retrofitting a conventional diamond with a DDI have contributed to increased use of the DDI. Research on DDIs has focused primarily on the assessment of operational benefits. Formal safety evaluations of DDIs are lacking. This study aimed to fill the knowledge gap by conducting a safety evaluation of DDIs using three types of before–after evaluation methods: naive, empirical Bayes, and comparison group. Three evaluation methods were used because each involved different trade-offs, such as data required, complexity, and regression to the mean. All three methods showed that a DDI replacing a conventional diamond decreased crash frequency for all severities. The highest crash reduction (59.3% to 63.2%) was observed for fatal and injury crashes. Crashes involving property damage only were reduced by 33.9% to 44.8%. Total crash frequency also decreased by 40.8% to 47.9%. A collision diagram analysis revealed that the DDI, as compared with a diamond, traded high-severity crashes for lower-severity crashes. Whereas 34.3% of ramp terminal fatal and injury crashes in a diamond occurred in left-turn angle crashes with oncoming traffic, the DDI eliminated this crash type. One potential concern for the DDI is the possibility of wrong-way crashes, but only 4.8% of all fatal and injury crashes occurring at the ramp terminal of a DDI were wrong-way crashes. The DDI offers significant crash reduction benefits over conventional diamond interchanges.