"What a difference a year and a half makes," Napa County's transportation guru Mike Zdon told supervisors Tuesday.

He was talking about the approval by the Board of Supervisors on Tuesday of a spending plan, which will accompany a half-cent sales tax, if it's approved by voters next June.

Previously, the tax concept did not garner the full support of the board, and some representatives of the cities of the county also looked askance at the tax.

Since then, meetings with so-called stakeholders and elected officials have led to broad-based support assuring a spot on the June ballot.

Over 30 years, the tax will raise $537 million with 28 percent of that going to Jamieson Canyon improvements, 6 percent to transit needs and the lion's share to local street and road improvement and maintenance.

Napa is near the bottom of the list when it comes to Bay Area road quality. Zdon said this will improve that situation greatly.

Last week, the financial pot was sweetened for American Canyon, leading to City Council support of the plan after it had been teetering on opposition.

In order to get many of the former opponents to come on board, officials agreed to include a discussion of land use and growth issues in the mix, including the extension of Measure J, which requires voter approval of changes to agriculturally designated lands.

It will also see the county encouraging cities to adopted their own vote-approved Rural Urban Limit lines which would help contain growth within currently urban areas.

"This is a much improved measure," said Supervisor Bill Dodd, who has worked long hours to break down the barriers surrounding the transportation tax. "This is a big day for Napa County."

Supervisor Brad Wagenknecht, also a former skeptic, said he could now support the plan noting that Chair Diane Dillon and Dodd, once at odds over the tax issue, now worked in concert on it.