The new proposal will undergo more public review before a final decision is made.
If the plan goes through, it will take effect in the 2014-15 winter season. Next winter's season would operate under the current limits of up to 318 snowmobiles and 78 snow coaches.
Yellowstone Superintendent Dan Wenk said the new proposal is unique because it's based on the number of "transportation events" and not the actual numbers of snow machines. A transportation event is initially defined as either one snow coach or on average a group of seven snowmobiles.
The idea is to keep snow machines in groups, making it easier to control the overall noise and air pollution in the park and allow flexibility on the number of machines allowed to operate in the park.
"By the fact that we're basing this on the number of events and not the number of vehicles, it requires packaging of traffic which we think benefits greatly wildlife and natural soundscapes," Wenk said. "By packaging traffic we believe that we will be less impactful on the natural environment, and noise and air pollution testing will continue in the park to ensure that we stay within the requirements that we have."
Under the concept, the park can allow up to 480 snowmobiles plus 60 multi-passenger snow coaches on busy weekends, but the snowmobile groups cannot average more than seven snowmobiles over the entire season.
Jack Welch, a leader of the snowmobile advocacy groups called the Yellowstone Task Force and the Blue Ribbon Coalition, said snowmobile guides like the package concept.
"The bottom line is, they're able to run their business in such a manner that they can accommodate more people on certain days and average it out over the season," Welch said.
In addition, snow coaches for the first time will be subject to emissions and noise standards. And snowmobiles will be required to have stricter emissions and sound standards that Wenk said would further reduce the air pollution and noise they emit.
The stricter air pollution and noise standards for snowmobiles and snow coaches were key to the plan's success, said Mark Pearson, conservation program director with the Greater Yellowstone Coalition, a conservation group in Bozeman, Mont., that focuses on the park.
"We've always wanted them to adopt a plan that protects air quality and wildlife and soundscapes while still allowing for a reasonable amount of access, and I think we're hopeful that this plan will do that," Pearson said.
The agency's proposal also calls for keeping open the eastern entrance to Yellowstone, which typically requires crews to use artillery to trigger avalanches on Sylvan Pass to allow safe passage to the park for snowmobilers from Cody.
Pearson said it would be better that the pass was closed during the winter, but he recognized that the Park Service faced much opposition to the idea in Wyoming.
Wyoming Gov. Matt Mead said keeping access to Yellowstone in the winter is an important part of the state's economy.
"This plan strikes a balance that I support and worked to achieve," Mead said in a statement. "It will have long-term beneficial impacts on the economy and the people of Wyoming."
The Park Service has been wrestling with snowmobile access to Yellowstone in the winter for more than a decade. Previous plans have been met with a slew of court challenges and environmental appeals, causing confusion among park visitors and concern among communities surrounding the park that rely on snowmobile rentals for income.