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Diverging diamond interchange at Union Road & 120 Bypass is opening in 19 days
union interchange
This is a scene from an animation video on the City of Manteca’s website of the Union Road/120 Bypass diverging diamond interchange looking to the west.

California — the government entity widely credited for the birth of the modern freeway system — is adding a French twist.

And it’s debuting this month along the 120 Bypass in Manteca when traffic starts flowing Wednesday, Nov. 25, across the first diverging diamond interchange ever built in California.

The $28.4 million interchange at Union Road is funded by redevelopment agency bonds, the countywide Measure K one cent sales tax for transportation projects, and growth fees.

The design entails Union Road traffic doing a double cross. By doing so it reduces conflict points for accidents from 26 on a conventional design down to 14 on a diverging diamond. Besides the potential for improved safety it keeps traffic flowing at more efficient levels.

The world’s first diverging diamond interchanges were two constructed in Versailles in France in the 1970s. French engineers were tasked with increasing the capacity of an existing interchange without requiring the demolition of nearby development.

The result was the diverging diamond design.

Manteca Councilman Gary Singh has driven across a diverging diamond interchange and he didn’t have to go to France to do it. Instead he drove on the Moana Lane diverging diamond interchange on Interstate 580 in Reno.

“You don’t even realize you are on (what would normally be) the wrong side of the road,” Singh said.

Singh said it looks confusing looking at animation videos from above. But at the ground level driving across the interchanges it is easy to follow, Singh noted.

Councilman Dave Breitenbucher has crossed a diverging diamond interchange in Medford in Oregon on Interstate 5. It is one of 112 in the United States and 124 worldwide.

“It took a little getting used to,” Breitenbucher said.

Breitenbucher, a retired City of Manteca fire captain, is looking forward to seeing how the design pans out for improved safety.

The reduction in conflict points and the virtual elimination of T-bone accidents can cause the most serious injuries has been noted in other locations as a big plus. The design eliminates left turns at traffic signals.

Breitenbucher is also looking forward to auxiliary lanes being opened in connection with the new Union Road interchange between Airport Way and Main Street to help ease congestion on the 120 Bypass until work is completed on the 120 Bypass/Highway 99 interchange upgrades. That Caltrans project is scheduled to start a year from now.

There will be an auxiliary lane from the Union Road on and off ramps in each direction to the Main Street and Airport Way interchanges.

Singh noted the City Council first started pursing the diverging diamond design to save money by not requiring the exiting bridge deck demolition and the taking of nearby homes and property to put in place a half diamond interchange.

Then, as elected leaders became familiar with the concept, the safety advantages and improved traffic flow became strong selling points as well.

The overall cost of the diverging diamond project is coming in at $10 million less than if the city had pursued a traditional design.

Singh also is pleased the project includes the first protected pedestrian bridge crossing of the 120 Bypass in Manteca.

 That part of the project is funded largely by a Measure K grant.

The design that includes tunnels under freeway ramps and an easy loop design that is Americans with Disabilities Act compliant — allows pedestrians, bicyclists and those in motorized wheelchairs to cross the freeway without having to contend with off and on ramp traffic that often fails to look for  people on foot. The design also provides a physical barrier to separate traffic form pedestrians on the bridge deck. That is something that did not exist on the old Union Road overpass. There is also no sidewalks or barriers on the Airport Way and Main Street bridges to safeguard pedestrians.

The pedestrian portion also includes stairways on either side that allow those on foot to circumvent the loops.

The city is working toward deploying similar designs for upgrades at existing interchanges at Airport Way and Main Street.

Critics have argued that Airport Way should have gone first.

But when work started on the project in 2007, the big issue was traffic that would be generated by the 757,000-square-foot Promenade Shops at Orchard Valley anchored by Bass Pro Shops that would open in 2008. At that time, Costco wasn’t in the picture nor was other development that is now along the Airport Way corridor.

The city committed to the Union Road project on the reasonable assumption the developer would lease the in-line space. After the Great Recession hit and slowed down retail and an effort to convert the space into factory outlets only attracted four tenants, Poag & McEwen became difficult to deal with for restaurant chains as well as those wanting to rent retail space to work with leading local leaders to believe the firm was using the center as a tax write-off.

The reformatted firm is now pondering building apartments and condos where part of the parking lot is with the goal of making it a mixed use project similar to Santana Row in San Jose.

The revised interchange will open just as the first part of a major shopping complex on the southwest corner of the interchange anchored by the 116,641-square-foot Living Spaces furniture showroom has started.

Had Manteca shifted the money committed to Union Road to Airport Way it would have thrown well over $2 million in environmental and engineering work out the window. It would have then had to spend at least three years and spend even more money to focus on another location.

It may not be too long after the Manteca project opens that work could start on the state’s second diverging diamond interchange.

The next interchange is planned at Hatch Road on Highway 99 in Ceres.

The City of Manteca has posted an information video on how the diverging diamond interchange operates on its website site. It can be accessed at


To contact Dennis Wyatt,