A Comprehensive Safety Assessment Methodology for Innovative Geometric Designs

Innovative geometric designs are often considered as a solution to the challenge of meeting the increasing travel demands with limited recourses (FHWA, 2009). This study focuses on one such design, the diverging diamond interchange (DDI), which aims to improve traffic flow and reduce congestion at highway junctions (Schroeder et al., 2014); but the methodology proposed is transferable to other geometric designs. Utah was among the first states to consider the DDI as a viable interchange option. The state opened its first DDI at the intersection of American Fork Main Street and I-15 in August 2010. Empirical studies have demonstrated the operational and cost benefits of DDIs (Bared et al., 2006; MoDOT, 2011; UDOT 2012; Yang et al, 2014); however, the safety impact remains inconclusive. Theoretically, the DDI design offers a safety benefit because it reduces the number of conflict points in comparison to other interchange options, which can lead to fewer crashes in general. Moreover, the lower design speeds in DDIs may also result in fewer and less severe crashes. One major safety concern with DDIs is that drivers may stay to the right at the crossovers and accidentally enter the opposing lanes. Despite the theoretical safety benefits, little research has been undertaken to quantify the safety impact of DDI using real-world crash data, primarily because of the limited accident history available. A preliminary safety study (MoDOT, 2011) directly compared the crash rates before and after the construction of a DDI in Missouri and concluded that total crashes dropped by 46% in the first year of operation. However, the simple before-and-after method assumes that any changes to the safety performance can be attributed solely to the DDI design. In reality, confounding factors that change continuously, such as traffic flow, traffic composition, and weather conditions, can also affect the safety performance. Therefore, the project proposes an alternative approach to deal with possible confounding factors by comparing the safety performance of DDIs with that of a group of reference sites. In addition, most existing safety studies on novel geometric designs only investigate vehicle crashes. Nevertheless, to fully understand the overall safety impact, the project also needs to take pedestrian and cyclist safety into consideration. Since a DDI usually attracts more traffic to the interchange, pedestrians and cyclists may feel intimidated by this increase and they may take detours to avoid the interchange. Hence, pedestrian and cyclist crashes may migrate from the interchange to nearby intersections. As a result, the project also proposes to examine all crashes involving pedestrians and cyclists within a one- to three-mile radius of the interchange and comprehensively evaluate the true safety effects of DDIs. Utah has been a pioneer and leader in adopting innovative interchange and intersection designs. Currently, the state has six operating DDIs and more are under construction or planned. Four DDIs in the state have been opened to traffic for more than two years, which provides sufficient accident data for a comprehensive safety study. This study will be one of the first independent studies in the nation to investigate the overall safety impact of DDIs. The results will be useful in evaluating DDI construction and retrofit projects in Utah as well as other states. The research is expected to have a broad and significant impact on the implementation of innovative interchange and intersection designs.


  • English


  • Status: Completed
  • Funding: $80000
  • Contract Numbers:


  • Sponsor Organizations:

    Research and Innovative Technology Administration

    University Transportation Centers Program
    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Managing Organizations:

    Mountain-Plains Consortium

    North Dakota State University
    P.O. Box 6050, Department 2880
    Fargo, ND  United States  58108-6050
  • Project Managers:

    Kline, Robin

  • Performing Organizations:

    Utah State University, Logan

    Civil and Environmental Engineering Department
    Logan, UT  United States  84332
  • Principal Investigators:

    Song, Ziqi

    Chen, Anthony

  • Start Date: 20150609
  • Expected Completion Date: 20180731
  • Actual Completion Date: 20180910
  • USDOT Program: University Transportation Centers Program
  • Source Data: MPC-480

Subject/Index Terms

Filing Info

  • Accession Number: 01579597
  • Record Type: Research project
  • Source Agency: Mountain-Plains Consortium
  • Contract Numbers: DTRT13-G-UTC38
  • Files: UTC, RiP
  • Created Date: Oct 23 2015 3:15PM