Manteca opened California’s diverging diamond (DDI) at Union Road and the 120 Bypass.
San Bernadino opened the state’s second DDI at Interstate 215 and University Parkway this week
Vallejo this year is expected to finish work on the Golden State’s third DDI at Fairgrounds Drive and Highway 37.
Tracy is on target to complete the fourth in 2028 at Interstate 580 and International Parkway/Patterson Pass Road
Lathrop is in the preliminary study stage of a DDI project at Interstate 5 and Louise Avenue.
And now Manteca has committed serious change to eventually having the only place in California with three consecutive DDIs within a mile of each other on a freeway.
The City Council on Tuesday approved a $3.6 million contract with Mark Thomas and Co. to conduct project approval and environmental documents for DDI projects at Main Street as well as Airport Way.
The firm did the same work for the Union Road DDI.
The city already has spent $500,000 on a pair of project study reports (PSR) — the first step in California’s process to pursues the freeway interchange projects.
Caltrans approved the PSR for the Airport Way DDI on April 14, 2025. Eight days later, the agency gave the OK for the PSR on the Main Street DDI.
The San Joaquin Council of Governments granted Manteca a $500,000 grant to cover the two initial studies and $3 million to go toward the work on the studies just awarded.
The $3.5 million comes from set aside funds for regional congestion relief generated by the countywide half cent Measure K sales tax for road and transportation projects.
The balance of the cost came from growth fees collected by the city.
The two new DDIs will have six thru lanes instead of four as is in place at Union Road.
Both will be designed with a grade separated ribbon path to enhance bicycle and pedestrian safety as is in place at the Union Road interchange.
The city is not likely to line up funding at the same time to break ground concurrently on both projects.
If that is the case, the Airport Way DDI will be constructed first.
Advantages of DDIs
The Manteca decision to go with the state’s first DDI at Union Road reduced the cost of converting the original two lane over crossing into a four lane interchange as a partial cloverleaf by $10 million.
The savings were reflected in being able to incorporate the original bridge deck into the overpass and avoiding the need to condemn nearby homes and property to accommodate the footprint of a partial cloverleaf.
Other advantages of a diverging diamond interchange according to Advanced Transportation Solutions/America are:
*Fewer conflict points (14 for diverging diamond design, 26 for conventional design). That results in fewer accidents.
*Better sight distance at turns.
*Traffic calming features when desired.
*Additional right of way is rarely needed.
*Virtually no driver confusion based on a study and observation of existing diverging diamond design interchanges.
*Construction time is reduced.
*Wrong way entry to ramps is extremely difficult.
*Maintenance of traffic is simplified during construction.
One drawback is pedestrian access requires at least four crosswalks (two to cross the two signalized lane crossover intersections, while two more cross the local road at each end of the interchange).
Manteca addressed that concern by creating a separate crossing for pedestrians and bicyclists that goes underneath the ramps and crosses the freeway separated from traffic using the equivalent of concrete K-rails.
Another “drawback” is free-flowing traffic in both directions on the non-freeway road is impossible as the signals cannot be green for both directions simultaneously.
That said, the design improves the flow movements to and from the ramps to avoid backups onto the freeway that can be an issue for the 120 Bypass
To contact Dennis Wyatt, email dwyatt@mantecabulletin.com
