If Napa wants to get serious about combating global warming, it should be approving higher density developments that mix housing with stores and jobs, a transportation expert recommends.

Compact development reduces the need for roads and vehicle trips, a major contributor to greenhouse gases, said Reid Ewing, a research professor at the National Center for Smart Growth at the University of Maryland.

Ewing addressed more than 30 people at Napa City Hall Wednesday night as part of a speakers series intended to raise public awareness of local transportation problems and solutions.

The Napa County Transportation and Planning Agency is updating its transportation plan, a process that could lead to a second attempt, in November 2008, to pass a transportation sales tax.

Ewing touted the environmental benefits of coordinating transportation and land use projects so that people can walk or reduce the distance they have to drive to meet daily needs.

The city of Napa generally scores points for staying within defined boundaries, Ewing said. Napa has begun approving higher density housing, with plans to combine housing with commercial development in downtown and on nearby Soscol Avenue.

Ewing said he was a contributor to a soon-to-be-released report by the U.S. Environmental Protection Agency that will tout the benefits of so-called smart growth to curb global warming.

If the U.S. had a strong policy that discouraged urban sprawl, the growth in greenhouse gases from vehicle emissions over the next 50 years could be cut by as much as half, he said.

Ewing conceded that many people do not like higher densities. Changing their opinion will involve "a lot of public education," he said.

Fortunately, there are many examples of neighborhoods made better when higher density development is accompanied by good design, he said.

As the public becomes more concerned about global warming, the politics of growth will change, Ewing said.

"I think that the pressure to develop land differently will rise to the point that elected leaders will say we have no choice," he said.

America will become "a little more European," Ewing said.

Several audience members asked if it made sense to expand south county roadways to handle congestion, much of it from people who can't afford Napa housing but work here.

Wider highways are sometimes needed, but communities generally cannot pave their way out of congestion, Ewing said. As much as 70 percent of new highway capacity will be gobbled up by locals who decide to drive again at peak hours or travel more, he said.

Free-flowing highways also encourage inefficient, sprawling development, Ewing said. "If you can drive faster, you will drive farther," he said.

Ewing encouraged politicians to build more housing that workers, especially those with lower incomes, can afford here. The best way to lower the cost of housing is to allow greater densities, he said.

The next speaker in the NCTPA series will be James Corless, who runs the Metropolitan Transportation Commission's Transportation for Livable Communities and Housing Incentive Program.

Corless will speak on June 20 at 8 p.m. in the Council Chambers at Napa City Hall.