The use of Diverging Diamond Interchanges (DDI), also referred to as Double Crossover Diamonds (DCD), has become more prevalent throughout the United States over the past 3 to 5 years. Overall, DDIs are gaining momentum within the interchange design community. However, only a limited amount of guidance on the design of these types of interchanges exists. The first DDI in the US was constructed in 2009 by the Missouri Department of Transportation (MoDOT). The DDI design accommodates left-turning movements at signalized, grade-separated interchanges of arterials and limited-access highways while eliminating the need for left-turn phasing. On the arterial, two-phase traffic signals are installed at the ramp terminal intersections to shift traffic over to the left side of the roadway between the nodes of the interchange. Once on the left side of the arterial roadway, vehicles can turn left onto limited-access ramps without stopping and without conflicting with through traffic.
Currently, there are no existing guidelines/standards for design of this type of interchange. The design is extremely dependent on site-specific conditions which are demonstrated by the variations in the design elements of the DDIs constructed to date. Additionally, there is relatively little accident history available as no DDIs existed in North America prior to 2009. Analysis of the actual safety and operational performance of DDIs has been limited to a handful of in-service DDIs. An assessment of the current design process and safety analysis is needed to ensure that recent advances and emerging issues are appropriately leveraged and/or reflected in design practices, guidelines, and policies.
The research objective is to identify, review, and evaluate the geometric design features and the associated safety and operational performance of in-service DDIs across the US. This information will then be utilized to develop recommendations for the AASHTO Technical Committee on Geometric Design for consideration as future geometric policy and guidelines. Two basic questions will be addressed: (1) What are the essential design characteristics of a DDI and how should an engineer utilize this information in the design of a DDI and (2) What are the safety and operational benefits of utilizing a DDI and how might the designer utilize this information in their design?
Tasks in this project would include: (1) Perform a critical and comprehensive review of available data relevant to DDI design. (2) Discuss lessons learned with DOTs with in-service DDIs such as MoDOT, UDOT, KYTC, NYSDOT, TDOT, etc. in order to gain their perspective on DDI design and related safety and operational issues. (3) Assess whether, and to what extent, the current design process reflects the explicit consideration of performance (e.g., pedestrian treatments, safety) and promotes efficient, if not optimal, combination of design elements to yield designs that are cost-effective when considering life-cycle benefits and costs. (4) Develop geometric design criteria and guidelines for Diverging Diamond Interchanges. (5) Develop a work plan to analyze the information obtained from the research. (6) Execute the work plan following prioritization and approval from the panel. (7) Develop a report to document the research and provide recommendations for future geometric design policy and guidelines for design of DDIs.
A. It is expected that this research will be incorporated into NCHRP Project 03-113, "Guidance for Traffic Signals at Diverging Diamond Interchanges and Adjacent Intersections.”