The eastbound off-ramp and westbound on-ramp for Union Road at the 120 Bypass may be closed for 10 months to save $500,000 and reduce construction time of the new diverging diamond interchange by four months.

The Manteca City Council will consider a plan to do just that when they meet Tuesday at 7 p.m. at the Civic Center, 1001 W. Center Street.

The proposal, if implemented, will eliminate periodical ramp closures.

The need to look at ways to reduce the cost was triggered by delays caused by PG&E in relocating power lines. Because PG&E did not honor its agreed to schedule, it impacted the construction contract for the project which in turn incurred additional costs for the city.

The engineering team involved with the interchange project looked for ways to reduce costs.

One was to reduce the need to import soil by using existing ramps on the west side of Union Road. Caltrans agreed to the change.

In addition shortening the construction schedule by four months by closing the ramps for 10 months will save additional money.

At a community meeting on the subject, residents attending supported the move on a 22-1 vote.

If the council concurs it means the interchange will be completed by January 2021 instead of May 2021.

For 10 months those traveling east on the 120 Bypass will have to exit at either Main Street or Atherton Drive. The other two interchanges will be used by those on Union Road wishing to head west on the Bypass while the ramps are closed.

In a related council item, elected leaders will be asking for federal funding to help with replacing existing interchanges at Airport Way and Main Street with diverging diamond interchanges as well.

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.

The need for less traffic signals also reduces delays on Union Road as well as existing the freeway. That means better traffic flow and less pollution from idling vehicles.

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com