One of the most compelling benefits of the DDI design is that it increases safety. It does so by reducing the number of potential conflict points through eliminating left turns crossing opposing traffic. This can equate to as much as a 50% reduction in crashes.

A diverging diamond intersection also offers operational and cost benefits.
The DDI:

Careful analysis revealed that the DDI would provide a number of benefits at this site.

In an area of rapid building, the DDI option would comfortably fit within the existing Right of Way (ROW). There would even be opportunity to create surplus ROW, pulling the diamond ramps inward, toward the freeway.

DDI, with the crossovers under simple two-phase signal control, would provide a good level of service. By avoiding the complexities of left turn signals phasing, it would be easier to obtain progression through the corridor. And because left turns would not need to find gaps in
opposing traffic, safety is vastly improved with the DDI.

Pedestrian access is also easily accommodated in the DDI design. Using signals at the ramp terminals connected electronically with those at the crossover provides safe crossing phases. The split ramp terminal geometry, which uses a median to guide traffic, offers the additional
benefit of reducing the pedestrian’s crossing length.

The greatest concern regarding the use of the diverging diamond design centers on the human factor. The DDI functions well in France, where it was first installed but the question remains: can drivers in the US navigate a diverging diamond, the intersection that would force them to drive on the left side lane?

Yes.

To evaluate the how this unique design would work with unfamiliar drivers, the Missouri Department of Transportation (MoDOT) partnered with the Federal Highway Association (FHWA) to test the Kansas City DDI before construction. FHWA built a simulation of the proposed DDI design in its Highway Driving Simulator (HDS). The FHWA Human Centered Systems team then observed volunteer participants driving through the simulated interchange. The simulator did reveal some minor problems with sight distance, which were quickly corrected in the
design. Most importantly the simulator assuaged fears that drivers would bear to the right at the crossovers, despite signs and markings indicating otherwise. No drivers followed the wrong path at crossovers. Other types of driver errors occurred with the same frequency as with conventional interchanges.3

Based on their experience in the HDS simulation of the intersection, the Missouri DOT personnel decided to recommend the DDI design over an alternative traditional design.

The testing by MoDOT and FHWA worked to allay the most significant user concern about the diverging diamond interchange, but how does the design accommodate increased capacity needs? At Michigan’s Macomb County site, the Road Commission has future plans to expand
the 26 Mile Road corridor to a 6 lane boulevard, improving east-west county mobility between M-53 and I-94. Analysis determined that the DDI could easily accept additional lanes. By leaving space in the median, additional lanes could be added to the inside without impact to
the ramp terminals. Unplanned expansions could also be accommodated by adding outside lanes, but this would require reconstruction of the ramp terminals.

In the end, MDOT choose Option #2, the conventional diamond with roundabouts at the ramp termini for Macomb County’s M-53 at 26 Mile Road interchange, finding the consistently high Level of Service rating and very low user delay compelling characteristics. Still, the OHM engineering team is confident that DDIs will find a home at a Michigan interchange in the near future.

Michigan is not alone in its search for more effective traffic facilities. At least six other states are currently examining DDIs for specific sites:
Missouri, Oregon, New Mexico, Kentucky, Maryland and Ohio.

The diverging diamond interchange is one of the novel intersection designs that fulfill traffic planners’ often competing objectives of increased safety and capacity, with decreased congestion and infrastructure costs. With all these benefits, it won’t be long before Michigan
drivers discover diamonds in their own backyards.

For more information about diverging diamond interchanges, contact Stephen Dearing, PE, PTOE, Manager of OHM’s Traffic Engineering Services at 734/522-6711. Email: stephen.dearing@ohm-advisors.com.