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South county commute traffic will get incrementally worse with the approval of each new development, a report concludes.

Each planned airport area development project adds only incrementally to rush hour delays, but the cumulative impact is something else again.

The Napa Valley Transportation Authority addressed this issue in a new environmental study for its proposed bus maintenance yard. The yard is to be relocated from the city of Napa to Sheehy Court in the airport industrial area so it has room to grow.

Buses heading to and from their new home would add only a second or less to rush-hour traffic delays in the area, the study found. That blink of an eye isn’t enough under state law to require mitigation steps.

But the bus yard isn’t the only project in the works. Traffic number-crunchers added up expected delays from the planned Montalcino hotel and golf course, Napa Gateway Plaza phases one and two, Napa Bottling Center, Zapolski Rudd Winery, Gateway Winery, Napa Executive Management and Turnkey Technologies.

The result is that the dribs and drabs of traffic increases can add up, creating greater delays along Highway 29, the Napa Valley’s transportation lifeline to the Bay Area and Central Valley.

The Highway 29 and Highway 221 intersection – Soscol Junction — is already an evening rushhour traffic bottleneck, with commuters on average spending 67.8 seconds waiting at the signal. Add all of the approved airport industrial area projects and the delay will top 80 seconds, the environmental study found.

Eighty seconds in the transportation world is considered the threshold at a signalized intersection where “delays” become “excessive delays.” Good high school track runners can cover more than a quarter-mile in that time.

Morning commuters have it somewhat better. The average delay should rise from 44.8 seconds to 51.2 seconds, the study said.

The planned Soscol Junction solution is to build either a flyover or a series of roundabouts. The NVTA and the state Department of Transportation have been working on the $40 million project, but have yet to secure construction funds.

“That will be our No. 1 highway focus,” NVTA Executive Director Kate Miller said.

At the nearby, unsignalized Devlin and Soscol Ferry roads intersection, a morning commute wait of 37.5 seconds will top 80 seconds when the planned airport industrial area projects are built, the study said. Average evening commute waits already top this mark.

At intersections without traffic signals, delays topping 50 seconds are considered “extreme,” according to the Transportation Research Board.

Installing traffic signals at Devlin and Soscol Ferry roads could cost $250,000 to $500,000, county Public Works Director Steven Lederer said. He doesn’t see that as being a wise investment in an era when transportation funds are tight.

That’s because building the nearby Soscol Junction project could also go a long way toward fixing the Devlin and Soscol Ferry roads delay problem. That’s where the focus is, Lederer said.

“As long as that’s on the table, that’s the ultimate solution,” he said.

If Soscol Junction improvements don't get built before the Napa Pipe development opens, Napa Pipe would pay for new striping and other changes - but no traffic signal - at the the Soscol Ferry and Devlin roads intersection to improve traffic flow, Lederer said.

County studies showed that, until the Soscol Junction project removes the traffic signal there, a traffic signal can't be installed at Soscol Ferry and Devlin roads because the two intersection are too close to each other.

Another bottleneck is at Highway 29 and Airport Boulevard/Highway 12. Morning rush-hour commuters face an average delay of 51.9 seconds. This will rise to 70.7 seconds when already approved projects in the area are built, the study said.

Miller talked of extending Devlin Road to create a parallel corridor to Highway 29. She mentioned expanding Highway 29, though she added this will be expensive.

As it stands, commuters could see those dribs and drabs of traffic increases before the big-ticket, more comprehensive road improvements are built.

This story was modified from the original version to add clarifying comments from county Public Works Director Steven Lederer

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Napa County Reporter

Barry Eberling covers Napa County government, transportation, the environment and general assignments. He has worked for the Napa Valley Register since fall 2014 and previously worked 27 years for the Daily Republic of Fairfield.