California’s First Diverging Diamond Interchange Provides Safer, More Economical Option for City of Manteca
The First of Its Kind in the Golden State: City of Manteca and Caltrans Work Around Material Needs and Delays to Build Innovative New Interchange
When the City of Manteca, California, looked at improving the interchange at State Route 120 and Union Road, traditional designs proved too expensive for the redevelopment funds devoted to the project. To reduce costs, the city decided to build the state’s first diverging diamond interchange (DDI), which allowed them to stay within the existing footprint and expand the overcrossing instead of replacing it.
In the DDI, vehicles traveling in both directions will cross to the opposite side of Union Road on the bridge over SR 120. The two-phase traffic operation improves safety, since left turns won’t need to clear oncoming traffic.
The new configuration also increases capacity and efficiency, a necessary requirement with SR 120 serving as a major connector between the Central Valley and the Bay Area, while Manteca experiences significant residential and commercial growth. In addition, a new, grade-separated, Class I bicycle and pedestrian trail through the interchange will provide a safe route for increased mobility.
Although the California Department of Transportation (Caltrans) will maintain the interchange after construction, “The city took the lead in funding the project and pushing it forward because it meets their needs locally for circulation and development,” said Sam Sherman, Project Manager for Caltrans’ District 10. In addition to redevelopment bond funds, the city used Measure K funds and local development fees.
In 2014, the city awarded the $1.6 million design contract to Mark Thomas of Sacramento, California, after a quality-based selection process. General Contractor Teichert Construction of Stockton, California, was awarded the $23.7 million construction contract in March 2019 through a low-bid process.
Construction started last June on four new ramps to connect Union Road to SR 120. Utility delays required the team to devise a new construction staging plan, with hopes of completing the project in early 2021.
Why a DDI?
When the city identified the need for an interchange improvement in 2009, they started with a traditional, partial cloverleaf design. “The challenge was that the redevelopment bond funds were only about one-third of what we needed to build that,” said Mark Houghton, P.E., Manteca’s Director of Public Works. “Initially we looked for additional funds. We were on the brink of going out for bid when the recession hit and there was no more money.”
As a result, “We had to back up and rethink in terms of something more economical,” Houghton continued. “Mark McAvoy, our Senior Transportation Engineer at the time (now Director of Public Works for the City of Monterey Park, California), suggested a diverging diamond interchange and I said, ‘What’s that?’”
Although this will be the first DDI in California, Sherman said 95 DDIs had been built or were in progress nationwide as they evaluated the option in late 2015. In fact, shortly after McAvoy proposed the idea, Houghton happened upon a diverging diamond interchange in Utah, then another in Atlanta when he traveled there on vacation.
“I thought, ‘Wow, this could work,’” he said. “Performance-wise, the diverging diamond is very efficient. It can move a lot of traffic because it’s just a two-phase operation. There’s very little delay even in moderate to heavy traffic.”
The design also simplified construction. “With the DDI, we’re able to widen the existing overcrossing structure as opposed to having to replace it for a traditional interchange design,” said Matt Brogan, P.E., Mark Thomas’ Principal in Charge. “That saved the city over $8 million.”
In addition, the compact footprint of the DDI eliminated the need for right-of-way acquisition. “In the initial design, the ramps were closer to the local roads,” Sherman said. “The diverging diamond interchange allowed us to pull those ramp termini with Union Road closer to SR 120 and farther away from the local road intersections, which also improved operations.”
For drivers, “The DDI is inherently a much safer design,” Brogan said. “It takes 26 conflict points in a traditional interchange and reduces it to 14. Because of the free left turn that comes on and off the diverging diamond, it eliminates broadside, left-turn collisions. Reduced speeds through the interchange also eliminate the potential for high-speed collisions.”
Standards on the Go
As this project began, several other DDIs were in environmental review throughout the state and Caltrans was in the process of developing standards. However, their Design Information Bulletin (DIB) wasn’t finalized until December 2017, several years into Mark Thomas’ design process.
“That slowed the project down a bit,” Sherman said. “We used the Federal Highway Administration’s guidelines, draft guidelines from Caltrans, and some other information Mark Thomas had.”
Ultimately, “We took a lot of the lessons learned from our design development and worked with Caltrans’ staff to implement those into the DIB,” Brogan said.
Digging Holes and Hauling Loads
The other part of the project design, the new Class I bicycle and pedestrian trail, connects residential developments on the south side of the interchange to commercial and mixed-use developments on the north side.
“One of the big questions with a DDI is how do you get people across?” Houghton said. “It’s not normal vehicular flow, so Mark Thomas created a crossing independent of traffic.”
The one-mile trail crosses the Union Road structure then loops down under each of the SR 120 entrance ramps. “We grade-separated the trail through the interchange footprint,” Brogan said. “That eliminated all conflict points with vehicles so it’s much safer.”
Four concrete slab bridges provide vehicle access on top and bike and pedestrian facilities underneath. “The contractor is building up embankments so the interchange ramps accommodate the bridges,” Brogan said.
Because Teichert needs to build the new embankments while keeping existing ramps open, they developed a plan to minimize materials hauled in and out. “We dug some holes to utilize materials on the site,” explained Alex Salcedo, Teichert’s Senior Project Manager. “When we close the existing ramps for good, we’ll essentially push that material into the holes. Unfortunately, there’s not a lot of room on this project so we’re only digging a few holes, and we can’t go too deep because the groundwater is pretty shallow. We dug down about 10 feet in one area and 12 feet in another.”
Those restrictions mean Teichert needs outside materials, too. “Ultimately the project will need a total of 270,000 cubic yards to complete,” Salcedo said. “We ran 400 to 500 loads a day to bring in about 140,000 cubic yards of material in the initial phase. With our limited space, we had to get the trucks in and out of the site quickly.”
To accomplish that, “We put some traffic signals in a flash condition during the day to keep the trucks moving,” Sherman said. “Teichert also used special pads and other measures to shake excess dirt off the truck tires and keep it off the roadway.”
Making Up Lost Time
Unfortunately, utility delays disrupted the project’s original schedule. In order to widen the bridge, Pacific Gas & Electric needs to relocate two, 60,000-volt power poles. “We gave them notice to relocate those by the end of October 2019, but it looks like it will be late January or early February 2020,” Sherman said.
The team worked together to find an alternate staging plan that balanced public convenience with the owner’s cost, settling on longer-term ramp closures to expedite the work. “If we didn’t restage and close some ramps, there would be at least a six-month delay,” Salcedo said. “By doing this, we can make up at least four months of time.”
Before the DDI opens in 2021, the city will conduct public outreach and education. “It will take some adjustment from the community, but if you just follow the lines, you drive right through the interchange and may not even realize you were on the other side of the road,” Houghton said. “When you tie together all the benefits of the DDI – economy, functionality, and safety – I think this interchange will provide great service.”