Motorists within a few months will see the start of what will end up being more than four years of major road work on a 5-mile segment of the 120 Bypass.

It involves three interchange projects with a combined price tag of $107.4 million.

The first project to start will be the $23.7 million diverging diamond interchange being built at Union Road. Work is expected to begin within the next month or so now that PG&E, Caltrans and the city have come up with a deal that allows Manteca to pay for the relocation of four major transmission power lines that are in the project’s right of way. The additional $1.8 million cost to relocate the poles is in addition to the contract that the council has approved to award to Teichert Construction.

Then next year the city is on target to go to bid and award a contract to start construction to create an interchange at McKinley Avenue for $30.7 million. The project is now fully funded with the complete local match expected to be collected by 2020. It could be pushed back a year which means higher construction costs if the city is unable to negotiate a lower payment to the state as part of a program to protect the threatened California Tiger Salamander that was found on the project site recently. That could cost the city as much as $1.4 million on top of the $30.7 million projected cost of the actual interchange.

The first interchange work involves the 120 Bypass/Highway 99/Austin Road improvements. That three-phase project will cost $131.5 million overall. Funding has only been identified for the first phase that will cost $54.4 million. The goal is to award the contract in the summer of 2021, start construction in the fall of 2021, and have work on the first phase completed by the fall of 2023.

That first phase would:

widen the connector from the eastbound Bypass to southbound Highway 99 to two lanes.

replace the existing Austin Road overcrossing with a new bridge that would also would go over the Union Pacific Railroad tracks.

swing Woodward Avenue south to connect with Austin Road.

relocate the current at-grade Woodward Avenue crossing of the railroad tracks to reach Moffat Boulevard so vehicles cross the tracks at 90 degrees. This would be a new, short road that only goes from the new alignment of Woodward Avenue to Moffat Boulevard.

temporarily shut down the southbound off ramp and northbound on ramp at Austin Road and the Highway 99 interchange.

build a new northbound off ramp to Highway 99 at Austin Road.

The configuration would allow traffic to use Moffat Boulevard — which would flow under Austin Road — to access southbound Highway 99 as it currently does.

The biggest impact of the Union Road work on 120 Bypass traffic flow will be an auxiliary lane that will be constructed on both sides of the freeway from Union Road to both Airport Way and Main Street. That would allow for longer mergers and help reduce the impact on traffic flow from local motorists who will jump on the freeway at one interchange and exit at the next.

The diverging diamond interchange — the first in California — that will be built at Union Road has lanes crossings to the opposite side freeway where the ramps are and then crossing back over at the ramps on the other side of the bridge.

Where the traffic crisscrosses there 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.

McKinley Avenue will originally be built as a partial cloverleaf.

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com