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State’s first diverging diamond interchange includes 120 auxiliary lanes for longer merges
A rendering of what the diverging diamond interchange at the 120 Bypass and Union Road will look like.

Commuters on the 120 Bypass will appreciate Manteca building California’s first diverging diamond interchange at Union Road even if they never drive across the bridge deck where travel lanes cross each other twice to improve flow and reduce the potential for accidents.

That’s because the $23.7 million project includes constructing auxiliary lanes from Union Road along the 120 Bypass to both Airport Way and Main Street in both directions to allow a much smoother weave in and out of freeway traffic. Ramp metering signals will also be installed.

The auxiliary lanes will help ease some of the stop-and-go congestion that plagues the 120 Bypass that is the deadliest and most accident prone stretch of freeway in the Northern San Joaquin Valley where, on average, an accident is happening almost every day.

The City Council on Tuesday is expected to award a $23.7 million contract to Teichert to do the work. The four bids ranging from $23.7 million and $29.6 million came in over the engineer’s estimate of $20 million. The second lowest bid by DeSilva Gates Construction was just under $53,000 more than the Teichert bid. When the council meets at 7p.m. at the Civic Center, 1001 W. Center St., they are also being asked to award a $2.3 million contract to Drake Haglan & Associates for construction management and inspection services. That is in addition to a $604,729 contract with Mark Thomas & Company for construction staking and support services.

To allow the project to go forward staff is recommending switching $2.2 million of the funds set aside to build an interchange at McKinley Avenue and the 120 Bypass as well as $1,225,000 budgeted to extend Milo Candini Drive from where it now ends at the northern edge of the Big League Dreams sports complex to Yosemite Avenue to the project.

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 then 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.

Staff not only came up with the solution but was able to secure Measure K sales tax funds to pay for the separate pedestrian bridge.

The fact two tunnels beneath ramps are involved prompted staff to think of possible safety concerns. In some cities where such pedestrian tunnels are in place they have become spots after dark for criminal acts and for the homeless to bed down.

As a result the tunnels will have:

uSecurity cameras placed and protected so they can’t be damaged to provide live feeds backs to the Manteca Police Dispatch Center.

uThere will be extensive lighting that will also be placed and protected in a manner where they can’t be damaged.

uThere will be a 24-hour emergency button tied in directly to the 9-1-1 system.

uIt will be equipped with a device that emits continuous noise that is extremely uncomfortable to hear for an extended period of time.

Why the diverging

diamond design was

selected by Manteca

The diverging diamond approach was recommended by Public Works Department staff after they were tasked by elected leaders to try and come up with the most cost effective upgrade for the interchange.

The only other requirement was to move larger volumes of traffic.

Originally the plans called for a half cloverleaf that would have required taking nearby property including several homes.

Staff looked into diverging diamond designs that were deployed in other states that were selected not for cost savings as much as it was to place interchanges that worked effectively in developed areas without taking more land and to improve safety.

In the case of Manteca the design means a project price tag that was $10 million less as no additional land was needed. Also a cloverleaf style interchange would have required demolition work resulting in a longer construction frame.

As an added bonus the design allows for faster and smoother traffic flow and — based on observations of diverging diamond interchanges in place — less severe collisions as well as fewer accidents.

Caltrans District 10 has helped clear the way for the design that has been deployed in 80 plus other locations in the country — with the nearest being on Mona Lane in Reno

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.

To contact Dennis Wyatt, email