Jamieson Canyon Road is considered the poster child for Measure H, the transportation sales tax on the June ballot.

This 6-mile roadway is notorious for its twice-daily traffic crawls, punctuated by occasional head-on collisions. Commuters and shoppers who use the canyon to get to Interstate 80 live with uncertainty about traffic conditions and fear of accidents.

If Measure H passes, motorists can expect safe, clear sailing through Jamieson Canyon within eight years, backers say.

The two-lane state highway would expand to four traffic lanes, with a center median barrier and bicycle lanes. Cross-traffic would be restricted to six locations.

Sounds like motoring nirvana, but the picture is more complicated than that. Nearly half of Jamieson Canyon Road is in Solano County, which has its own Measure H sales tax on the June ballot.

Critics worry that Napa could pass its transportation tax while Solano does not. Could motorists end up traveling halfway down Jamieson on four lanes that narrow to two at the county line?

What if gridlock merely moves a few miles east to the juncture of Jamieson Canyon and I-80?

Critics also question Jamieson Canyon's safety threat. The fatality rate on another local highway, Highway 12/121 through Carneros, is four times higher.

Despite such criticisms, officials with the Napa County Transportation Planning Agency and the Solano Transportation Authority insist Jamieson Canyon is appropriately Measure H's star project.

Jamieson Canyon -- due to heavy traffic, high crash rates and critical importance to the Napa economy -- consistently ranks high in voter polls as a must-do project, said Mike Zdon, the local transportation planning agency's executive director.

There is no chance that just half of Jamieson Canyon will be widened, said Daryl Halls, the Solano Transportation Authority's executive director. If only one of the half-cent sales taxes passes, elected leaders in the two counties would likely cobble together an alternative funding package, he said.

If the Solano measure passes and the Napa tax fails, Napa could potentially borrow future state highway allocations to fund its portion of the $114 million project, Zdon said.

This might be Napa County's only major highway project for the next 30 years, but the widening would get done, he said.

This is why passage of Measure H is so important, Zdon said. Without a local transportation tax, Napa County doesn't have the funds to partner with Caltrans to build essential highway improvements, he said.

Lacking a local transportation tax, Napa County waited a quarter-century to accomplish the $57 million Highway 29/Trancas Street interchange, Zdon said.

Saving up annual state stipends to accomplish Jamieson Canyon improvements could similarly consume local resources for decades to come, he said.

Measure H solves this financial stalemate by raising $150 million for what is called the Jamieson Canyon Corridor, while plowing another $356 million back to cities and the county for local street and road improvements.

With Caltrans and Solano County also bringing money to the table, Jamieson Canyon would be widened and two interchanges would be built, at Highway 29 and Jamieson Canyon and another where 29 meets 221 (Soscol Avenue).

Napa County would sell bonds guaranteed by Measure H revenues over the next 30 years to accomplish these projects by 2014, Zdon said.

A do-it-now project

Jamieson Canyon can't wait, Zdon said. The roadway now carries 32,000 vehicles a day. By 2030, the number is projected to be 60,000, he said.

Who is congesting Jamieson Canyon?

During the peak rush hour, the road carries 7,500 vehicles an hour, with an estimated 5,166 being commuters coming to work in Napa, Zdon said. Most of the outflow are Napa residents going to jobs in Solano or adjacent counties, he said.

At other times, including weekends, the roadway is clogged with visitors coming to Napa Valley wineries and restaurants and locals going off to shop and recreate, Zdon said.

Solano County intends to widen its share of Jamieson Canyon while simultaneously contributing to a billion-dollar Caltrans plan to unsnarl the I-80/680 interchange, Halls said.

There are interim projects that should help traffic onto and off of Jamieson Canyon at I-80 short of the final solution, Halls said.

A leading Measure H critic, Leon Brauning, argues that other roads have higher collision rates than Jamieson Canyon. Measure H isn't doing much to help those more dangerous locations, he says.

Citing Caltrans statistics, Brauning said the Carneros Highway has a significantly higher fatality rate, but no Measure H money will be spent there. Silverado Trail had 10 fatalities in two years, yet H is largely silent about this important north-south road, he said.

Zdon concedes that Measure H will not fix every problem, but say it's a beginning. He notes that Caltrans is about to begin an $8 million safety project on Highway 12/121 that will soften two curves and widen shoulders, giving motorists on the Carneros Highway an extra margin of safety.

As for Silverado Trail, Napa County will have a pot of Measure H money that the Board of Supervisors can allocate to the Trail, Zdon said.

There are no easy solutions to traffic deaths on Silverado Trail, said Don Ridenhour, the county's assistant public works director. They happen along the entire 25-mile roadway, although most occur at the southern half, he said.

The county has talked about installing a rumble strip like the one on Jamieson Canyon Road, while continuing to be "aggressive" about requiring left-turn pockets for new wineries, he said.

If Measure H passes, the county will be able to install Silverado Trail safety measures sooner, Ridenhour said. The intersection of Silverado and Yountville Cross Road is a candidate for improvements, he said.

Besides reducing crashes, a widened Jamieson Canyon with a median barrier would prevent frequent closures that cause traffic havoc at a major gateway to Napa County, Zdon said.

There are nearly two crashes a week on Jamieson Canyon, which often result in traffic being detoured south onto American Canyon Road, which isn't designed for that vehicle load, he said.

If motorists are unhappy with today's conditions on Jamieson Canyon, they should use American Canyon Road, Brauning said.

Congestion and dangerous conditions on Jamieson Canyon are prime motivators for the business community to support efforts to pass Measure H.

Employers increasingly draw workers from Solano County, where housing is both more plentiful and less expensive than in the slow-growth Napa Valley.

Dennis Sisto, CEO of Queen of the Valley Hospital, said his board of trustees endorsed Measure H because so many hospital employees depend on Jamieson Canyon.

"There just isn't enough workforce in Napa County. We're seeing more workers come from Solano County," said Sisto, who said 26 percent of Queen staffers come from Solano.

In addition to widening Jamieson Canyon and building two interchanges on Highway 29, Measure H would provide $2 million for express buses between Napa and Fairfield and another $2 million to promote carpooling.

To counteract concerns that current growth control measures would be insufficient if Jamieson Canyon becomes a free-flow highway, county officials are asking cities to consider voter-approved municipal boundaries. Napa already has this slow-growth measure, while Napa County has several comparable defenses in place.