On a busy summer day in Zion National Park, 16 propane-powered shuttle buses do the work that 5,000 cars once did.

The shift to shuttle-only traffic through the heart of the park eight years ago is praised by park officials and locals for its role in cutting congestion, pollution and visitor frustration.

But park officials say the shuttle system could face cutbacks if it doesn't get additional money in the coming years.

If funding remains at 2008 levels, costs could exceed revenue by 2012, according to the first in-depth analysis of Zion's shuttle program.

Cutbacks could include reduced hours, longer waits between rides and cutting out certain service areas, the analysis said.

Jock Whitworth, Zion's superintendent, said long-term funding for the shuttle program remains the biggest challenge.

"We definitely want it to keep going," he said. "I think the Zion shuttle is spectacular."

The shuttle program began in 2000 after years of complaints from frustrated visitors who couldn't find a place to park in the vehicle-clogged Zion Canyon, which features some of the park's most popular features, such as Angels Landing and Court of the Patriarchs.

The shuttles also reduce smog, noise and roadkill.

Last year, nearly 3 million people used the shuttles. Recent park-sponsored surveys indicate that more than 95 percent of visitors like the system. The buses are free and arrive at eight designated stops about every seven minutes inside the park.

"It's viewed as very much a model of what the Park Service stands for," said Kevin Percival, who manages the agency's transportation planning.

Shuttles are used in other national parks, too, including Grand Canyon, Alaska's Denali and Acadia in Maine.

Percival said lessons learned in Zion and elsewhere are being closely watched by other park managers looking for alternatives to car, trucks and RVs.

Zion officials say a bus full of visitors is the equivalent of keeping 28 cars off the road. They estimate that carbon dioxide emissions are cut by 12 tons a day.

"It has allowed a lot more people to get into the canyon and experience it," Whitworth said.

The March-to-November system, which also includes nine buses that run between the nearby town of Springdale and the park, costs about $3 million a year, Whitworth said.

Costs are rising, including the price of propane over the last year, he said.

The fleet is getting older too. Park managers have taken to rehabilitating some of the aging buses and extending their lives by about six years, Whitworth said. But there are additional upcoming costs: replacing or rehabilitating the entire fleet could cost between $6 million and $12 million.

In order to keep the system running as is, supplemental funding will be needed, according to the park's analysis.

Whitworth said he doubts that a fee hike at the park will be considered anytime soon. He said park officials are working with Springdale in search of a way to raise more money. Those talks are in the early stages.

Rick Wixom, Springdale's town manager, said the shuttle program has widespread support among businesses and residents, even though some were skeptical when it began.

"When it stops running in November, it's sort of like something's missing in the town," Wixom said.