Diverging Diamond Construction Brings Design-Build to Delaware
An innovative interchange design that is new to the state of Delaware - and not yet that common across the country - is moving closer to completion. Under the auspices of the Delaware Department of Transportation (DelDOT), a diverging diamond interchange, or DDI, is under construction at the junction of State Route 1 and Route 72. The $7.5 million project includes the design and construction of a modification to the existing SR1/SR72 interchange from a diamond interchange to a diverging diamond interchange - the design-build project is being constructed by Diamond Materials of Wilmington and was designed by JMT of Newark.
Although based on a standard diamond interchange design, DDIs eliminate left turns that have to clear opposing traffic, making the design useful in high-traffic areas. Within the interchange, traffic briefly drives on the left side of the road to allow left turn movements to occur without crossing oncoming traffic or stopping. A DDI has fewer conflict points, reducing the opportunities for crashes, and there is greater capacity for vehicles at the interchange.
According to DelDOT, a DDI reduces the number of ways vehicles can collide by almost half. The design also results in greater capacity and efficiency, since it can accommodate more traffic than conventional designs. At intersections where there is a high volume of left turns onto the Interstate, DDIs reduce traffic backups because the free-flow left turns mean vehicles do not have to stop to access the ramp. Drivers are guided with overhead signs, pavement marking and traffic signals.
The Missouri Department of Transportation installed the first DDI in the country in Springfield in 2009. Currently there are only approximately 64 DDIs in operation or design, in 48 of 50 states. DelDOT officials visited the Missouri project while planning their new SR1/SR72 interchange.
Interchange in Need of Major Improvements
The completion of a State Route 1 Widening Study indicated the need to improve congestion and safety related to the heavy left-turn movements entering and exiting the route, reports DelDOT Project Engineer and Program Manager Darren O'Neill. "It's really an interchange project with three intersections. The through movement on SR72 is currently around 20,000 vehicles, and the project area's level of service is very poor in peak hours due to high left turn movements. Thus a diverging diamond will provide a viable solution with limited area impacts. The project is designed for projected 2040 traffic volumes of 28,000 to 30,000 vehicles."
On April 29, Delaware Governor Jack Markell and DelDOT Secretary Jennifer Cohan attended the project's groundbreaking. At the event, Governor Markell commented, "Implementing the diverging diamond concept on our network of roads and bridges represents the efficient and effective approach to transportation investments that our citizens deserve. We will improve safety, while using an interchange design that can be installed at a quarter of the cost of other methods. This is exactly the type of project we wanted to see when we worked with the General Assembly to approve a $400 million increase in transportation funding over the next six years."
Said DelDOT Secretary Cohan, "When I became Secretary of Transportation, I made a commitment to innovation, and the Diverging Diamond Interchange is one of the most innovative concepts in road design. It's a design that has been successfully applied in other states, and is recognized as a cost-effective method for improving traffic flow and safety without widening an existing overpass."
The Federal Highway Administration awarded DelDOT an Accelerated Innovation Deployment (AID) Demonstration grant in the amount of $1 million, which helped fund the DDI project. The grant provides funding as an incentive for eligible applicants to accelerate the implementation and adoption of innovation in highway transportation. Grant recipients are encouraged to utilize the monies to promote ways of improving the work of highway planning, design, construction and operation.
The project was developed during the fall of 2014; the Request for Qualifications process was completed in the spring of 2015, and the Request for Proposal process in the early fall of last year. The design-build contract was awarded in December, naming JMT as the designer and Diamond Materials as contractor.
Founded in 1971, and headquartered in Sparks, Maryland, Johnson, Mirmiran & Thompson (JMT) provides a full range of multi-disciplined engineering, architectural, and related services to public agencies and private clients. Founded primarily to perform engineering services in support of land development projects, in 1979 JMT entered the public works market by focusing on transportation engineering for Federal, state and local public agencies.
Diamond Materials is located in Newport, and serves Northern Delaware, parts of Chester and Delaware Counties in Pennsylvania and Salem County, New Jersey. The company provides a complete and full line of site construction services, asphalt products, and demolition of buildings and bridges.
O'Neill says that the project is expected to be functionally complete in November or December. "Of course, this assumes that we'll have a normal late fall instead of an early winter. By the end of the year, the new alignment should be open to traffic, but there may be temporary striping, etc."
Challenges of a Unique Project
With a unique, uncommon, and unfamiliar design, public resistance and other challenges would have been expected in bringing Delaware's first diverging diamond interchange - and the state's first interchange developed through a design-build process - to reality. DelDOT's O'Neill says, however, "We expected resistance, but it didn't happen. Of course, public support of a DDI was a crucial need, and a public workshop process was critical.
"The design-build process does present its own challenges. For one thing, you have some parts of the project still in the design stage while other parts are already under construction. And there are challenges during construction that you do not have with more conventional interchange design, such as the need to maintain traffic during design and construction."
Breanna Kovach, DelDOT Project Engineer and Project Development Engineer, adds, "Design-build is not that common for DelDOT. It's a different way of approaching a project - we give the criteria, but the design-build team can meet those criteria in the way they choose.
"One unique aspect--typically, when building a new road project, that project is separate from the existing traffic. But with DDI, the construction is right there. The positive is that there is less impact on the traffic and a smaller footprint. But the negative is there is traffic and a smaller footprint."
William Schaub, JMT Vice President, adds his own perspective on the challenges: "This is our first diverging diamond interchange project. One challenge was meeting the schedule requirements, especially with no standard specifications for DDIs. The standard protocols weren't applicable to this project. Additionally, the pace of design-build projects is much quicker."
Josh Crane, Project Manager for Diamond Materials, reiterates, "Not only is this the first DDI in the state, it's also Delaware's first design-build interchange project. We've been trying to build as much of the new configuration as we can without impacting traffic. And with this kind of design, you have to ensure that you are matching up your vision with that of the designer, so it's been helpful that JMT is very knowledgeable."
To prepare drivers for use of the new and unfamiliar interchange, DelDOT has utilized several educational efforts, O'Neill reports. "We developed outreach and workshops, and sent out a postcard mailing showing the completed interchange with directional alignments, in advance of the project."
One deliberate step, adds O'Neill, was the used of standard signage along the new interchange. "In their meetings with us about their experience with the DDI, the Missouri officials were adamant that unique signage shouldn't be used - since it would call out the fact that traffic is coming into something different. Instead, they recommended the use of standard signage, so the new interchange would be perceived as just part of the traffic flow."
Adds Kovach, "The geometry is such that it's a natural pattern to go through a DDI."
A Successful Team
The innovative diverging diamond interchange at SR1/Route 72 is poised to provide improved traffic flow, congestion relief, and safety to Delaware drivers along the busy route. While the design-build process for an interchange, and the DDI design itself, have not been that common in the state, the project has moved along at a steady pace thanks to the collaboration of the project partners. As JMT's William Schaub states, "The new interchange will certainly improve the operations of the interchange, increasing its operational efficiency. And working as a cohesive team has really been helpful on this project."