New Mexico DOT Embraces the Diverging Diamond
Contractors may be seeing more of an innovative form of highway interchange in coming years. The diverging diamond interchange (DDI) design, developed in Europe and now introduced at a few U.S. locations, has some distinct safety and cost advantages over the traditional design. The New Mexico Department of Transportation (NMDOT) is the latest to implement this innovative design, at a crossing near Santa Fe of the Interstate 25 and Cerrillos Road (NM 14).
Identifying a DDI
A Diverging Diamond Interchange is a variation of the standard diamond interchange commonly used where a freeway crosses a minor road. As in the standard diamond interchange, the freeway and other road are grade-separated, that is, the freeway is elevated above the road. With the DDI design, the twist, literally, is that the two directions of traffic on the non-freeway road cross to the opposite side on both sides of the bridge at the freeway. So traffic in that section is driving on the opposite side of the road.
The DDI allows for two-phase operation at all signalized intersections within the interchange. This improves safety, since no long turns have to clear opposing traffic.
The diverging diamond design originated in France, where three of the interchanges were built in the 1970s. The idea was revived and developed by experts in the United States and the first U.S. DDI was built in Springfield, Missouri, in 2009. Popular Science magazine listed the DDI as one of the best innovations in 2009 (engineering category) in "Best of What's New 2009." Since then, about 50 DDIs have been built in the United States.
Bringing the Interchange to Santa Fe
The Santa Fe interchange project began in May and is expected to finish in December 2016. Located on the south side of the City of Santa Fe, the Interstate 25 and Cerrillos Road (NM 14) interchange acts as a vital conduit for travelers between the city of Albuquerque and the Capital. The contractor on the $19.8 million project is FNF Construction Inc., based in Tempe, Arizona. Molzen-Corbin & Associates, of Albuquerque, New Mexico, designed the interchange.
Through the course of this project, FNF will completely remove five bridges, build two new bridges, and realign six ramps to convert the existing standard diamond interchange into New Mexico's first DDI. This new layout will provide a safer configuration while also easing traffic congestion.
A multi-use trail will also be constructed using partial alignment of a former ramp. Along this path will be three multi-plate tunnels with a cumulative length of 650 feet.
This project also features drainage improvements, earthwork, asphalt milling and paving, retaining walls, fencing, and lighting improvements to benefit the intersection and its users.
Upgrading the Interchange
The current design has been in service for over 40 years and was in need of a significant upgrade or replacement. Over the past 20 years, it had been shown to have major weaknesses, including traffic backups, bottlenecks, deteriorating bridges, and unsafe merging distances on ramps.
Habib Abi-Khalil, PE, Acting District Engineer for NMDOT District Five said a number of other options were considered for the interchange project, including a double roundabout, but none offered the same level of service, safety and cost-effectiveness. "We're not doing it to be the first," said Abi-Khalil, but the innovative designs the best for the specific site.
Benefits NMDOT expects for the Santa Fe DDI are improved safety and efficiency, reduced traffic congestion, longer merging lanes for getting on and off the Interstate, reduced length and cost of construction, and enhanced pedestrian mobility.
Abi-Khalil conceded that some people are leery of the idea when they hear of it, but said people who actually drive through a diverging diamond interchange hardly realize there is anything different about it.
Constructing a Non-Traditional Design
"As a contractor we have never constructed a DDI," said Matt Trembly, Senior Project Manager, FNF Construction Inc. "As far as construction is concerned the project has the same scopes of work as other interchanges. The main difference in doing the DDI is the phasing of the work and how to construct the interchange while minimizing the amount of traffic closures and interruptions."
Trembly said, "We are trying to minimize the amount of times we switch traffic so the traveling public knows where to drive and also to maintain an adequate traffic flow through the construction zone to minimize commuter delays."
The fact that there is a commuter railroad that runs through the middle of the project also adds an additional level of safety concerns to be addressed by the contractor. FNF has installed a permanent fence to separate the railroad safety zone from the work zone and future traffic zones. "This will improve overall safety for all commuters in the future," Trembly said.
Every construction project has its challenges, however. Trembly said the contractor had to contend with a quick start on the project. "It takes time to get subcontracts completed and submittals turned in and approved so different scopes can begin work," he noted.
An additional challenge was the removal of several prairie dog colonies in the construction zone. Trembly said FNF hired a specialist to humanely remove the prairie dogs and relocate them to other colonies inside the Santa Fe vicinity. The capture and relocation process took approximately two weeks to complete.
But for Trembly, what is most special about this project is that it is a signature location. "For many that commute from the Albuquerque area, this is the "gateway" to Santa Fe, and there are many commuters that travel through this project daily."
Along with that, the project also happens to have the main offices for NMDOT and the Federal Highway Administration just down the street, which means there are many interested officials driving through every day. "Then there is the State Capital building," Trembly added, "So there are also many politicians that drive through the project. With all of these officials and commuters driving through, we are a very high profile project."
As DDIs get more attention, more contractors will find themselves working on these projects. Abi-Khalil said, "The idea is spreading, I know California is looking into it." It's only a matter of time before a DDI is seen in every state.