Central Valley to Get a Diverging Diamond Interchange

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California’s first Diverging Diamond Interchange (DDI), a traffic design engineers say will reduce collisions and improve traffic flow, is coming to the Central Valley.

The DDI will replace an existing interchange on SR 120 at Union Road in Manteca, in San Joaquin County. That part of the city contains a popular shopping and entertainment center, as well as new housing development, and more growth is expected into the future. Improvements at this crossing have been in various planning stages for over fifteen years.

A recent groundbreaking ceremony marked the beginning of construction on the DDI, and it is expected to be completed by the end of 2020.

The DDI, said Caltrans District 10 Director Dan McElhinney at the groundbreaking, “slows everybody down but gets everybody mobile.” That is, it keeps cars moving through a complicated merging pattern instead of requiring them to stop at an old-fashioned intersection.

It also avoids interactions between cars and other road users by removing pedestrians and bicyclists from the roadway. Instead, engineers will build a long, looping, separated path for them, dropping down into a short tunnel to cross under the road and then climbing up to the interchange to cross over the highway at the same level as vehicle traffic.

The basic concept of a DDI is that motorists enter from the right side of the road, cross over to the left side as they go through the interchange, and then cross back again to the right to exit (see a simulation here). Engineers love the design because it supposedly improves traffic flow by removing the need for traffic signals at on and off ramps. They also say its diverging diamond shape will prevent congestion by reducing stopping points.

Koosun Kim, deputy director of Public Works for the city of Manteca, said one of the main reasons that a DDI design was chosen is because it is expected to reduce collisions. The existing interchange has 26 potential conflict points; the DDI design has only fourteen of them.

While proponents of DDIs tout safety for car occupants, others say that they demonstrate blindness to the actual experience of bicyclists and pedestrians trying to get across.

Kim said concern for the safety of pedestrians and bicyclists is why the city decided to build a separate path for them.

“If you look at what it’s like right now--it’s very dangerous for children,” he said. There is a school located on one side of the existing interchange and a shopping center and movie theater on the other. Children often cross the interchange on the only currently available space, which is the shoulder.

Usually DDIs locate pedestrian walkways in the median or along the sides of the road, and place bicycle lanes next to vehicle traffic. The Manteca DDI will instead include a separate twelve-foot-wide bridge on the east side of the overpass. To get through the interchange, pedestrians and bicyclists will have to drop down from street level, follow a tunnel under the freeway ramps, then climb upward on a circular path to the bridge, where they will cross over with vehicle traffic, but behind some kind of barrier. Then they will have to repeat the sequence, backwards, on the other side, as illustrated in the top image.

A staircase will be provided as a shortcut for pedestrians who want to avoid the longer ADA-compliant loop. Security measures, including cameras, lighting, and emergency call buttons, will be added to the tunnel.

The design is inspired by one at Highway 50 and Watt Avenue near the American River in Sacramento.

For bicyclists and pedestrians, the trip across the interchange will be about twice as long as it is for vehicles, and that much longer than the current, dangerous shoulder crossing.

The project will also encourage more and faster vehicle traffic in other ways. It will widen SR 120, building "auxiliary lanes" from Airport Way to Main Street, and widen Union Road to four lanes between Daniels and Lifestyle streets.

The city of Manteca is fully funding the project with a combination of Redevelopment Agency funds (the interchange improvement was assigned RDA money before the agency was dissolved), local development fees, and Measure K, the half-cent sales tax approved by San Joaquin County voters.

DDIs have been built around the country, but this will California's first. Manteca's design is expected to be the model for other California cities that are considering similar construction projects. There are at least four in the works: in Modesto, San Bernardino, Ceres, and San Diego.

Caltrans hopes so. “We hope to see many more like this,” McElhinney said at the groundbreaking.