Orange County Branch Newsletter

March 2018

ASCE OC Transportation & Development Institute

Delivering the State's First Diverging Diamond Interchange

By Ravi Shah, PE, QSP/D

The ASCE Orange County Branch in conjunction with the Transportation and Development Institute organized a luncheon with speakers from Mark Thomas regarding Delivering the State’s First Diverging Diamond Interchange for Manteca, CA at the State Route 120/Union Road Interchange. The speakers Rob Himes – President of Mark Thomas and Aaron Silva – Deputy Project Manager were critical to the development of the project from initial concept – where the project started from its current existing condition as a Caltrans L-1 interchange to original design concept as an L-1/L-7 and was finally developed into its current configuration as a Diverging Diamond Interchange. The development of the concept into its final configuration helped save the City of Manteca, millions of dollars in design and construction.


The speakers were engaging and used humor and their technical expertise to discuss the history of interchange design from Caltrans starting from inception in the 1950’s to the present. As they begun the development of the first Diverging Diamond Interchange (DDI), they worked closely with Caltrans to develop DIB #90. Their presentation outlined the key benefits of DDIs, which includes a significant increase in traffic operations and a decrease in conflict points when compared to Diamond Interchanges, Single Point User Interchanges (SPUIs) and DDIs. In short, a reduction of nearly half the conflict points between Diamond Interchanges and DDI’s.

Furthermore, the team came with an innovative solution for pedestrian and bicycle circulation from the traditional concepts that have been utilized on DDI’s through the US, which circulate pedestrian traffic through the middle of a DDI. Their concept included keeping pedestrian circulation on the outside and creating a separated bike path, which would facilitate a key connection point for bicyclist while greatly increasing safety. This concept utilized from another Mark Thomas project, showed these key connections can completely transform bicycle circulation in a positive manner - this was shown with bicycle circulation heat maps, which showed a new trend in bicycle circulation once a new bike route that enhances safety is constructed.

Finally, the presenters discussed key geometric design criteria differences between traditional ramp interchanges and those established and required as part of a DDI. These criteria were developed in conjunction with Caltrans as part of this pilot project with the City of Manteca. Overall, the presenters did an excellent job to briefly cover key issues and challenges while providing critical information on the successful delivery of the State’s first DDI.

It was evident based on the presentation that as DDI’s become more prevalent within California and as more are currently being proposed during Alternative development, one thing is certain – a DDI doesn’t work in every scenario. The development of a DDI is constrained greatly by existing conditions, traffic patterns and volumes and the proper evaluation of those constraints is critical to the success and adoption of this innovative interchange concept.

To view presentation slides, click here.

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