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Union Road-120 Bypass diverging diamond interchange improves traffic flow, saves $$$
A rendering of what a diverging diamond looks like. - photo by Courtesy of Illinois Department of Transportation

Two road projects — considered key to retail development as well as part of an effort to help ease traffic issues in South Manteca — are now funded and likely to break ground in 2018.
The Manteca City Council Tuesday approved a funding plan that will allow both the Atherton Drive gap between Union Road and Airport Way as well as the diverging diamond interchange at Union Road and the 120 Bypass to be built.
The diverging diamond interchange will be the first to be built in California.
The Atherton Drive project is expected to pull traffic off of Woodward Avenue as it would start functioning as the south Manteca thoroughfare allowing traffic to get off the 120 Bypass and to travel to where Atherton Drive now intersects Woodward Avenue near Moffat Boulevard.
That will especially be true when traffic tie-ups or accidents at the 99/120 interchange on the eastbound 120 Bypass prompts vehicles to exit the freeway and head down Woodward Avenue to reach Highway 99.
Developers — who are paying for the $4.4 million project from fees already collected — have indicated the fact the Atherton Drive gap will now be built is likely to result in a decision by retail firms to build freestanding locations fronting the 120 Bypass. It is a view shared by city officials.
At the same time Atherton Drive punched through to Airport Way will likely change traffic patterns for residents south of the 120 Bypass and even east Manteca. That’s because they could take Atherton Drive — designed as a thoroughfare with minimal intersections — to reach Costco and the Stadium Retail Center that is a top shopping destinations without getting on the 120 Bypass.
Councilman Gary Singh pushed for the city to look at placing traffic signals at Airport Way and Atherton Drive once the road gap is built. Councilman Richard Silverman argued in favor of waiting until such time traffic volume justified signals.

Union Road/120 Bypass
Interchange will be a
first for California
The diverging diamond design for the Union Road/120 Bypass interchange calls for traffic lanes crossing  on either side of the bridge structure so northbound traffic would cross the bridge on the west side instead of the east side with the southbound lanes on the east side instead of the west side. Once they clear the bridge they are switched back.
It is on schedule now to go to bid in early 2018 meaning it will be the first diverging diamond built in California There are three others in the design phase elsewhere in the state. The first diverging diamond was built in 2009 in Springfield, Mo. There are now 89 diverging diamonds in use nationwide.
“The diverging diamond takes dollars and cents and puts them to work effectively to improve traffic flow and reduce construction costs,” Manteca Mayor Steve DeBrum said.
While enhanced traffic flow and improved vehicle safety will result from the design, the council’s primary motivation in going with the diverging diamond was to maximum available dollars. Given the design does not need additional land and can employ the existing structure, it represents a savings of $10 million off the $25 million price tag that converting Union Road into a partial cloverleaf would involve.
DeBrum said if the Union Road interchange proves successful the city in the future could look at similar solutions when it goes to widen the Airport Way and Main Street interchanges along the 120 Bypass. Should that occur, Manteca would be able to upgrade three interchanges for the same price it would take to make two partial cloverleaf interchanges in constant dollars.
The on and off ramps along with the flipping of the lanes creates two semi-diamond shaped intersections on either side of the bridge. This eliminates the need for traffic from both directions on Union Road as well as that coming from the 120 Bypass avoid needing any  traffic signals to exit an off ramp or to get into an on ramp. The project will also include ramp meter signals such as now are in place on the Lathrop Road/Highway interchange.
There will also be auxiliary lanes put in place along the 120 Bypass between Union Road and the Main Street and Airport Way interchanges
The project will also include Manteca’s first separate pedestrian/bicycle bridge across the 120 Bypass to address growing concerns about safety as more and more walkers head across Main Street, Union Road, and Airport Way as housing develops south of the 120 Bypass. None of the existing overpasses have sidewalks or protected bicycle lanes. The diverging diamond design as employed in other states requires pedestrians and to cross into the middle of the bridge and then cross back to the edge at two signalized intersections.

To contact Dennis Wyatt, email