Study Details

Study Title: Safety Evaluation of Seven of the Earliest Diverging Diamond Interchanges Installed in the US

Authors: Hummer et al.

Publication Date: JAN, 2016

Abstract: Diverging diamond interchanges (DDIs) are increasingly popular because they provide improved traffic operations and cost savings. DDIs should be safer based on theory, but previous empirical safety studies have been limited. The objective of this work was, therefore, to conduct a broader safety evaluation of DDIs and recommend a crash modification factor (CMF) for the conversion of a conventional diamond to a DDI. The team analyzed seven of the earliest DDIs in the US. Four were in Missouri and there were also sites in Kentucky, New York, and Tennessee. The team collected over 28 site-years of crash and other data before conversion and over 19 site-years of data after conversion. The primary analysis was before and after with comparison sites to account for trends and potential simultaneous event biases. The results showed that crashes were reduced at most of the sites, and the team recommended a CMF of 0.67, meaning that installation of a DDI to replace a diamond should reduce all crashes by 33 percent. The reduction in injury crashes was even larger, with the team recommending a CMF of 0.59. Other analyses indicated that DDI installation should mean a substantial reduction of angle and turning crashes with some reduction in rear end crashes as well, although rear end crashes will still be the dominant crash types after DDI installation. Clearly DDIs offer potential safety benefits and agencies should consider them strongly as replacements for conventional diamonds.

Study Citation: Hummer, J.E., D., C.M.Cunningham, R.Srinivasan, S.Warchol, B. Claros, P. Edara, and C.Sun. "Safety Evaluation of Seven of the Earliest Diverging Diamond Interchanges Installed in the US". Presented at the 95th Annual Meeting of the Transportation Research Board, Paper No. 16-5481, Washington, D.C., (2016).


CMFs Associated With This Study

Category: Interchange design

Countermeasure: Convert diamond interchange to Diverging Diamond Interchange (DDI) or Double Crossover Diamond (DCD)

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
0.67 33 4 Stars All All Principal Arterial Other Freeways and Expressways Suburban
0.59 41 4 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Suburban
0.33 67 4 Stars Angle All Principal Arterial Other Freeways and Expressways Suburban
0.64 36 4 Stars Rear end All Principal Arterial Other Freeways and Expressways Urban
1.27 -27 4 Stars Sideswipe All Principal Arterial Other Freeways and Expressways Suburban
0.76 24 4 Stars Single vehicle All Principal Arterial Other Freeways and Expressways Suburban

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