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An Assessment of Safety and Geometric Design Criteria for Diverging Diamond Interchanges


The use of Diverging Diamond Interchanges (DDI), also referred to as Double Crossover Diamonds (DCD), has become more prevalent throughout the United States (US) over the past three to five 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. This effort will be coordinated with the National Cooperative Highway Research Project (NCHRP) Project 03-112, Spacing, Signal Timing and Performance of Diverging Diamond Interchange and Adjacent Intersections, which should begin in 2014.


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?

  2. What are the safety and operational benefits of utilizing a DDI and how might the designer utilize this information in their design?

Related Research:

Double Crossover Diamond Tech Brief, Publication Number: FHWA-HRT-09-054

*Date: October 2009, *http://www.fhwa.dot.gov/publications/research/safety/09054/

Drivers' Evaluation of the Diverging Diamond Interchange, *Publication Number: FHWA-HRT-07-048, * http://www.fhwa.dot.gov/publications/research/safety/07048/index.cfm

Alternative Intersections/Interchanges: Informational Report (AIIR), Publication Number: FHWA-HRT-09-060, http://www.fhwa.dot.gov/publications/research/safety/09060/

Diverging Diamond Interchange Performance Evaluation (I-44 & Route 13), Missouri Department of Transportation, February 2011, http://library.modot.mo.gov/RDT/reports/TRyy1013/or11012.pdf

Missouri DOT Engineering Policy Guide, Section 234.6, Diverging Diamond Interchanges, http://epg.modot.org/index.php?title=234.6DivergingDiamond_Interchanges

UDOT DIVERGING DIAMOND INTERCHANGE (DDI) OBSERVATIONS AND EXPERIENCE, Report No. UT-12.05, Utah DOT, April 2012, http://www.udot.utah.gov/main/uconowner.gf?n=10172614219775523

Missouri’s Experience with a Diverging Diamond Interchange - Lessons Learned, Missouri DOT, May 2010, http://library.modot.mo.gov/RDT/reports/UnNumbrd/or10021rpt.pdf

Chlewicki, Gilbert. “Learning From Eight Operational Diverging Diamond Interchanges in the United States”. TRB 92nd Annual Meeting, Washington, DC, 2013

FHWA Every Day Counts 2 – Intersection and Interchange Geometrics initiative which includes an operational and safety evaluation of DDIs and development of an informational guide based on data available as of Summer 2013.


Tasks in this project would include:

· Perform a critical and comprehensive review of available data relevant to DDI design.

· 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.

· 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.

· Develop geometric design criteria and guidelines for Diverging Diamond Interchanges.

· Develop a work plan to analyze the information obtained from the research.

· Execute the work plan following prioritization and approval from the panel.

· Develop a report to document the research and provide recommendations for future geometric design policy and guidelines for design of DDIs.

This research effort should directly involve the AASHTO Subcommittee on Design and Technical Committee on Geometric Design.


Establish new design and safety guidelines relative to DDIs.


The AASHTO Technical Committee on Geometric Design and TRB’s Geometric Design and Operational Effects of Geometrics Committees supports further research in this area and proposed this problem statement at their joint meeting held in Irvine, California (July, 2013).

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Bart Thrasher, Kent Belleque, Reza Amini, Marshall Elizer, Scott Wolf, Brian Toombs, Mike Wahlstedt, Jeff Shaw
Source Info:Problem statement developed as a result of the Safety Effects of Geometric Design Decisions Workshop at the 2013 mid-year meeting of TRB Committees AFB10 (Geometric Design) and AHB65 (Operational Effects of Geometrics), in conjunction with the AASHTO Technical Committee on Geometric Design. Problem Statement # 2015-G-23 on the NCHRP/AASHTO SCOR list.
Date Posted:02/07/2014
Date Modified:02/10/2014
Index Terms:Diamond interchanges, Highway design, Geometric design, Diverging traffic, Guidelines, Traffic signals, Missouri,
Cosponsoring Committees: 
Operations and Traffic Management
Safety and Human Factors

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