Manteca will debut a Highway 120 interchange unlike any you’ve seen in California
The state’s first “diverging diamond” interchange is set to open Wednesday on Highway 120 at Union Road in Manteca.
The design requires vehicles to cross briefly onto the opposite side of the Union Road bridge before turning onto the 120 on-ramps. Stoplights will control oncoming traffic.
Ceres plans to use the same concept for an interchange that could be completed by 2027 on Highway 99 at Service Road.
The design might confuse drivers unfamiliar with the idea, but advocates say it is safer and cheaper than a conventional interchange.
The Union Road junction is close to Bass Pro Shops and other businesses in south Manteca. Highway 120 also is a key link for travelers between the Bay Area and Sierra Nevada, and for Modesto-area commuters headed west on weekday mornings.
Teichert Construction built the interchange on a $23.7 million contract with the city of Manteca. It finished four months ahead of schedule.
Opening will be phased overnight
The interchange is scheduled to be fully operational by 5 a.m. Wednesday, after opening in stages over about five hours, spokeswoman Annette Manz said by email. The initial plan was to do it at 11:59 p.m. Wednesday.
Manz is public engagement manager for Dewberry Drake Haglan, a national firm providing construction management for the project.
Manteca’s funding includes the city’s defunct redevelopment agency and the extra sales tax approved by San Joaquin County voters for transportation.
Caltrans is overseeing the project because it involves a state highway. The agency has already been educating drivers with a video simulation based on a diverging diamond in Utah.
France built the first such interchange in the 1970s, according to a website called divergingdiamond.com. The United States got its first in Springfield, Mo., in 2009 and now has 99. The website was created by ATS/American, a transportation consultant based in Florida.
Ceres has same idea for Service Road
Ceres is doing detailed engineering on a 99 interchange serving the south end of town. Service Road is now just an overpass with no direct connection to the highway. The project would create these links via a diverging diamond.
The current Mitchell Road junction would be replaced with a partial interchange, with just a northbound off-ramp from 99 and a southbound on-ramp to the freeway.
The engineering will take until late 2022 or early 2023, City Manager Tom Westbrook said by email Monday. Construction would take another three years.
The project is expected to cost $133.5 million. Ceres plans to use $96.7 million in fees on developers, $31.1 million from the Measure L sales tax, and $5.7 million in redevelopment funds.
This story was originally published November 24, 2020 at 11:00 AM.
