Caltrans now has the blessing of the Napa County Transportation and Planning Agency to design a multi-level interchange at the junction of Highways 12 and 29 near the Napa County Airport.

A local citizen had suggested building a simple flyover for southbound traffic turning onto Jamieson Canyon Road. Because this design would be cheaper, it could be built sooner, saving money for other transportation priorities, Ginny Simms had told the board.

Ahmad Rahimi, a Caltrans project manager, shot down the flyover proposal Wednesday. After analyzing this concept three years ago, Caltrans concluded that a flyover would be inadequate to handle the higher volumes of traffic in coming decades, he said.

NCTPA directors unanimously endorsed Caltrans’ plan for a tight diamond interchange that would separate north-south traffic from east-west traffic.

If built today, the interchange would cost $61.4 million, Caltrans estimated. Costs will certainly escalate since there is no prospect of the interchange being built for years to come, Jim Leddy, NCTPA’s executive director, said.

Caltrans evaluated various designs in 2005, Rahimi said. The flyover concept was rejected because it cost a lot without solving the traffic problem, he said.

Simms questioned Caltrans’ analysis. Intuition said that a flyover would be much cheaper than a full-blown interchange, she said.

While a flyover might not be a perfect solution, it would allow traffic leaving Napa to turn onto Jamieson Canyon toward Fairfield without stopping, Simms said. Northbound vehicles on Highway 29 from American Canyon and Vallejo wouldn’t have to stop either, she said.

NCTPA could take the cost savings and apply it to finding ways to get people out of their cars, Simms said. The money could be used to improve transit and build park and ride lots, she said.

Even a small reduction in Highway 29 flows could significantly unclog the highway at peak periods, Simms said.

Simms, a former Napa County representative to the Golden Gate Bridge District, said traffic on the Golden Gate Bridge was greatly improved when transit options were expanded.

Supervisor Bill Dodd said Napa County needs to work to improve alternatives to cars while also building the interchange at the airport.

Because the county is sinking most of its discretionary highway funds into widening Jamieson Canyon Road, there won’t be any money for an airport interchange for many years, Leddy said.

Given today’s funding, it would be more than two decades from now.

Rahimi also said environmental studies for widening Jamieson Canyon from two to four lanes are done. Construction should start in the summer of 2010 and be completed three years later, he said.

The state is paying more than half the cost from bonds approved by California voters in November 2006. Napa and Solano counties are splitting the rest.