By allowing ads to appear on this site, you support the local businesses who, in turn, support great journalism.
Manteca breaks ground on state’s 1st diverging diamond interchange
Manteca and regional leaders break ground Tuesday on a project that will turn the Union Road interchange shown behind them into California’s first diverging diamond interchange.

It moves traffic quicker. It reduces the potential for traffic collisions. It takes less time to build. It costs a third less than a conventional interchange. And it is a rarity in that all of the work is being funded by local dollars.

The ceremonial ground breaking of California’s first ever diverging diamond interchange took place Tuesday morning in the JC Penney parking lot with the existing interchange structure in the background. 

The diverging diamond design that has been used in other states with great success came about after elected officials tasked municipal Public Works staff to come up with a solution to improve traffic movements through the Union Road and 120 Bypass interchange in the most efficient and cost effective way possible. The result — after an intense collaboration with Caltrans — was the diverging diamond interchange.  

The interchange that employs a bridge deck where travel lanes cross each other twice will be used as a model for similar interchanges through the state including the second one that will be built at Highway 99 in Ceres and a third now being considered in the Los Angeles Basin.

It is also highly likely the design that will be used in the future when Manteca goes to widen the 120 Bypass overcrossings at Airport Way as well as Main Street given it does not require a bigger footprint and the taking of additional right of way. It costs a third less than a conventional interchange project.

And while the $23.7 million project is designed for the more efficient movement of higher volumes of traffic on Union Road as the city grows, Caltrans District 10 Director Dan McElhinnery noted more drivers are likely to be impacted positively by something that was added to the project on the city’s dime and that of the San Joaquin Council of Governments — auxiliary lanes on the 120 Bypass.

The auxiliary lanes will run for a mile in each direction from Union Road to the Main Street and Airport Way interchanges. This will allow for a longer merge for traffic and should help ease the morning and afternoon commute congestion the 120 Bypass is notorious for.

“You will see a doubling of cone zones over the summer (compared to last year) thanks to SB1 funding,” McElhinnery said of the legislation that imposed the 12 cent gas tax hike with the promise taxpayers would start seeing almost immediately projects aimed at shoring up road infrastructure and aimed at easing congestion.

Mayor Ben Cantu noted it takes time to develop and fund projects on the scale of the interchange.

In a nod to previous elected city leaders Cantu acknowledged former Mayor Jack Snyder who was among those gathered to witness the ground breaking. He noted Snyder spearheaded the initial effort to secure state funding for the 120 Bypass decades ahead of schedule.

Cantu recalled how traffic in the 1970s and earlier would inch through Manteca on Yosemite Avenue that doubled as Highway 120 as people headed for the Sierra on Fridays and returned on Sundays. Manteca residents on those days often had to wait up to 15 minutes to cross Yosemite Avenue. It wasn’t unusual for traffic on the old Highway 50 that was replaced by Interstate 5 and Interstate 205 to be backed up to Tracy as it inched its way through Manteca on Fridays.

The interchange is being funded with $10 million in residual redevelopment agency funds, $12 million in growth fees collected on local construction and $2 million in countywide Measure K sales tax receipts.

The project will also allow a left turn lane to access The Promenade Shops at Orchard Valley from southbound Union Road midway between the interchange and Atherton Drive in front of JC Penney. That would allow motorists to access Bass Pro Shops, AMC Theatres and other stores from the interior road that runs along the 120 Bypass.

It also includes Manteca’s first freeway overcrossing designed exclusively for pedestrians and bicycles. A separate 12-foot wide bridge will be built to the east of the overpass with access to the bridge being via tunnels passing under off and on ramps with an ADA compliant loop leading up and down from the bridge. That will get pedestrian bicycle traffic away from the bridge deck.

The separate pedestrian crossing was the result of adamant pressure four years ago from Councilwoman Debby Moorhead and the Councilman Vince Hernandez and former Mayor Steve DeBrum who were growing frustrated with pedestrian safety in Manteca. It was the same year Manteca recorded two pedestrian fatalities.

Hernandez was perhaps the most passionate as he repeatedly pointed out fairly large numbers of people on foot and bicycle — many of them students — cross the 120 Bypass via bridges that have neither sidewalks, barriers separating them from traffic or fencing to prevent falls to the freeway below.

How diverging

diamond design works

With a diverging diamond interchange the flow across the freeway has lanes crossings to the opposite side of Union Road where the ramps are and then crossing back over at the ramps on the other side of the bridge.

Where the traffic crisscrosses they are traffic signals. On a traditional overpass turn movements on and off the freeway would also go through the traffic signals. That’s not the case with a diverging diamond interchange.

If Union Road was improved to a partial cloverleaf interchange as was originally envisioned there would be 24 conflict points for vehicles. The diverging diamond has 12.

Even more significant is the reduction in the potential for frequent T-Bone crashes that can result in extensive property damage and serious injury. There are 20 such conflicts on a traditional interchange and just two on a diverging diamond. Those two would be where the north and south lanes on Union Road crisscross.

Due to the interchange’s geometry the average speed is slowed from 40 mph to 25 mph.

Teichert Construction was awarded the bid for the project that will be completed sometime in 2020.

To contact Dennis Wyatt, email