In a case with broad national implications, the Sierra Club (search) wants to stop the widening of a freeway in fast-growing Las Vegas until the government proves the health of people living nearby won't be harmed by the automobile exhaust.

"We think the science is clear," said Jane Feldman, of the Las Vegas chapter of the Sierra Club. "There is a risk of cancer, heart disease and lung disease."

At issue is a five-mile stretch of U.S. 95 that would be widened from six to 10 lanes between the Las Vegas Strip and the well-to-do bedroom communities of Summerlin and Centennial Hills to the northwest.

The environmental group wants the 9th U.S. Circuit Court of Appeals (search) to order the Federal Highway Administration (search) to rethink the project. The court will hear the case Jan. 10 in San Francisco.

The Sierra Club already is looking down the road.

"This is the first case, I think, that raises the highway health hazards issue in litigation," said Joanne Spalding, a Sierra Club lawyer in San Francisco. "If the court agrees with the Sierra Club position, communities will raise this issue in any community with a heavily traveled highway."

A federal judge in Las Vegas granted the Sierra Club a temporary injunction last summer, preventing contractors from paving new lanes but allowing drainage and sound wall work continue.

Several hundred homes and businesses already have been demolished to make room for the widened freeway, and local, state and federal officials have criticized the Sierra Club for blocking what many call a crucial project to relieve gridlock.

"The 95 Corridor is the most congested highway corridor in the state," said Greg Bortolin, a spokesman for Nevada Gov. Kenny Guinn. "It's the No. 1 transportation priority in the state."

Federal Highway Administration project engineer Greg Novak disputed Sierra Club allegations that the environmental studies were inadequate and that the health threats from pollution were overlooked.

"We think we did it adequately. We apply the same rules in every state or city," he said. Novak said planners also considered a commuter rail line and buses before deciding the highway project was the way to go.

The stretch of highway was built in 1979 to handle up to 6,000 vehicles per hour. At the time, Clark County and Las Vegas had about 440,000 residents and the road ran to wide-open desert.

Today, southern Nevada has more than 1.6 million residents, and state transportation officials count 9,000 vehicles per hour choking the freeway during rush hour.

The project calls for adding two lanes in each direction, including the first carpool lanes in Nevada. The widening will allow the highway to handle 12,000 vehicles an hour by the end of 2006.

"Slowing this down just hurts the environment," Bortolin said. "It doesn't make a lot of sense to maintain a parking lot instead of a freeway."

The lawsuit counts 380 single-family homes, 27 apartment buildings, three schools, two community centers and a day-care center along the stretch of freeway

"We believe that widening the freeway for the convenience of commuters at the expense of the health of people who attend school and live immediately adjacent is unfair," Spalding said.

Spalding said she will ask the court to consider recent studies indicating that diesel fumes and auto-exhaust substances such as benzene and 1,3 butadiene raise the risk of cancer among people living near busy roads.

So far, though, neighboring homeowners have not mobilized against the project.

Dewayne Herrmann said he will be glad when the congested freeway is finally widened.

"If you don't have a wider freeway, you're still going to have the traffic, and it'll be more congested with more accidents because it's overloaded," said Herrmann, 53, a welder. "Cars standing still put out more pollution than if they're moving."