Editor's note: This updates with additional details from Friday morning's press conference, reaction from outside groups.

A preferred winter-use plan for Yellowstone National Park revolves around "transportation events" to control the daily numbers of snowcoaches and snowmobiles negotiating the park's roads. As such, at peak times there could be as many as 480 snowmobiles per day on Yellowstone's snow-covered roads, more than twice as many as have been counted in recent years.

However, the plan that now is subject to 60 days of public comment adds that the "maximum daily average number of snowmobiles per day would be 342 by the start of the 2017-18 winter season." That's a number still much higher than current usage levels, but one park officials believe will have minimal impacts on Yellowstone's air, water, wildlife, visitors, and employees.

Reaction to the preferred alternative was cautious Friday, as conservation groups applauded the direction Yellowstone officials were moving to in terms of protecting the park's airshed, soundscapes, wildlife, and visitors. However, they said the preferred alternative still needs some fine-tuning, particularly in terms of how many snowmobiles are permitted in a "transportation event," snowmobile emissions, and the intent to continue maintaining winter access across Sylvan Pass, with howitzers if necessary.

Similar concerns were raised by the Coalition of National Park Service Retirees.

“We support these essential elements of Yellowstone’s proposed plan,” said Maureen Finnerty, former superintendent of Everglades National Park and currently the chair of the Coalition's Executive Board.

“However, a portion of the plan contradicts the National Park Service’s emphasis that for snowmobiles to remain in Yellowstone, they must protect the park’s resources and values as effectively as snowcoaches," she added. "Overall, we believe this plan can serve Yellowstone well, but we urge a change where it does not stay true to its own goal and instead elevates the use of snowmobiles over Yellowstone’s conservation.”

How over-snow traffic in Yellowstone should be managed has been an issue that Park Service officials have grappled with for decades; concerted efforts to manage snowmobiles date to 1974 when the park adopted a master plan for winter use. The process became more complicated in the past dozen or so years, as lawsuits and threats of lawsuits sent Yellowstone planners back to the drawing board time and again.

The park has been operating under a temporary plan for the past three winter seasons. That plan expires next month, and without a new plan to replace it there would be no over-snow recreational access to Yellowstone beginning next winter.

The latest plan, which Yellowstone officials can be finalized in time to be implemented for next winter, would extend for one more winter the maximum daily allowance of 318 snowmobiles and 78 snowcoaches before implementing the approach to "transportation events" with the 2014-15 winter season.

That plan "is the most protective alternative that’s ever been proposed by Yellowstone or the National Park Service in the managed-use era," Yellowstone Superintendent Dan Wenk said. "It’s an impact-centric alternative. It makes the park quieter, and cleaner, than currently authorized. It develops new and very robust standards for both snowmobiles and snowcoaches."

Highlights of the plan's preferred alternative call for:

* Up to 110 "transportation events" per day; these events are defined as a group of seven snowmobiles or one snowcoach. However, the seven-snowmobiles-per-event would be the seasonal average; there could be times when as many as 10 snowmobiles are packaged in one group. Up to 50 of these events could involve groups of snowmobiles;

* Four non-commercially guided groups of five snowmobiles would be allowed each day of the winter season, one each through the park's Mammoth Hot Springs, West Yellowstone, South, and East entrances;

* Sylvan Pass would remain open for snowmobile and snowcoach access through Yellowstone's East Entrance, with park crews performing avalanche control as needed to ensure safe passage;

* Snowcoaches placed in service for the 2014-15 season, when the plan is scheduled to take effect, would have to feature "best available technology" (BAT) standards for their engine noise and emissions. By the winter of 2017-18, all snowmobiles and snowcoaches would have to meet the latest BAT standards.

Emissions studies outlined in the plan's supporting documents showed that BAT snowcoaches produced greater amounts of carbon monoxide per vehicle, while snowmobiles offering the latest in BAT -- sound levels at 67 decibels at cruising speeds vs. 73, and carbon monoxide emissions of 90 grams per kilowatt hour vs. 120 grams -- generated more hydrocarbons and nitrogen oxides both per vehicle and per person.

But trying to put those results in context was difficult for park planners, who noted that, "Without making a value judgment as to which pollutants warrant more concern relative to others, it is not possible to ascertain that one mode of transportation is cleaner or more desirable than the other or more protective of the park's air quality."

During Friday morning's press teleconference, Management Assistant Wade Vagias said there were several ways to interpret the air-quality data collected in the park. Under the park's preferred alternative that allows both snowmobiles and snowcoaches, 1.44 pounds of pollutants -- carbon monoxide, hydrocarbons, and nitrogen oxides combined -- would be generated per person, he said. The option to move all winter recreation traffic to snowcoaches would generate 1.92 pounds per person, pointed out Mr. Vagias.

Modeling also shows that existing carbon monoxide levels would remain at levels "well acceptable" to the Park Service and the Environmental Protection Agency, he added.

"While I will grant that looking at a representative round trip it would appear that snowcoaches are cleaner, particularly for NOX and hydrocarbons, less so for carbon monoxide, that's simply one filter for understanding the overall emissions picture," said Mr. Vagias.

Superintendent Wenk added that the comparison of emissions from snowmobiles and snowcoaches on a seasonal basis shows the two modes of transportation "are very comparable, and I would say that the comparison is very accurate on a season-wide basis, much more so than on a one-hour or eight-hour basis."

Concerning representatives of the Greater Yellowstone Coalition, the National Parks Conservation Association, the Winter Wildlands Alliance, the Sierra Club, and the Natural Resources Defense Council, as well as the Park Service retirees, were tables in the documents that show that groups of seven snowmobiles traveling together would generate eight times more hydrocarbons and 13 times more nitrogen oxides than a single snowcoach.

“Yellowstone has built its proposed plan around an assurance that ‘transportation events’ of different types will have ‘comparable impacts.’ But the park proposes to allow snowmobile groups to exceed its own measure of ‘comparability,’ generating disproportionate noise and exhaust and less ideal conditions for visitors seeking to experience and enjoy Yellowstone," said Deny Galvin, a former deputy director of the Park Service and current member of the Park Service retirees group.

"This is contrary to what law and policy requires in our national parks. It invites further uncertainty into the controversy over snowmobile use in Yellowstone National Park that has already stretched 15 years," he said. "We hope the National Park Service will correct this portion of its plan so that it comports with the plan’s many good components.”

Additional information contained in the documents showed there are quite a few existing snowcoaches at use in Yellowstone that are quieter than the sound limits the park is proposing -- no more than 75 decibels per snowcoach or group of seven snowmobiles at 50 feet. Ten of those sampled produced less noise; nine of those were at 73 decibels or less at cruising speeds.

Superintendent Wenk said it was decided to go with the 75-decibel limit in part because of the snowcoaches currently on the market.

"We established that based not only on the new machines that were coming into the process, but looking at the existing fleet and the retrofitting that would have to be done, and we determined that 75 was reasonable for across the fleet," the superintendent said. "We looked at what was reasonable to accomplish with snowmobiles, and we believe that the 67 dBA at cruising speed (reguired for all snowmobiles beginning with the 2017-18 winter season) was something that could be accomplished for snowmobiles, and 75 for snowcoaches."

Incentives aimed at encouraging tour operators to move more quickly to the latest snowmobile and snowcoach BAT standards could boost daily numbers slightly.

Short of those incentives, "At the highest potential level of use, with all 50 snowmobile events used in a single day and all snowmobiles meeting 'New BAT' requirements, there could be a maximum of 480 snowmobiles in the park," the executive summary to the final Winter Use Plan/Supplemental Environmental Impact read. "Although this is the maximum number of snowmobiles that could be permitted into the park on a single day, this level of use would not occur every day because commercially guided transportation events would be required to average no more than seven snowmobiles per event averaged seasonally, and non-commercially guided transportation events could not exceed a maximum group size of five.

"No later than December 2017, the maximum daily average number of snowmobiles would be 342 snowmobiles per day," the summary said.

Those numbers are considerably higher than actual numbers from the past three winters. While the park limit for over-snow vehicles has stood at 318 snowmobiles and 78 snowcoaches per day since the winter of 2009-10, the daily average up through the 2011-12 winter has ranged from 187 to 197 snowmobiles, while daily snowcoach numbers have ranged from 56 to 63, according to the park documents.

Superintendent Wenk said the decision to go with 110 "transportation events" was made based on "the number of transportation events that were currently authorized, and I thought that was a reasonable number ... 123 (under the park's definition of a transportation event) ... and I believe that I wanted to see if we could make the park quieter and cleaner."

The superintendent also is confident that by controlling over-snow traffic through "transportation events" there will be "less harassment of wildlife and less impacts and more natural quiet in the environment."

"I think it's going to allow the quality of the resource to remain excellent, and also visitor experience to be excellent," he told the Traveler on Thursday before the plan was released.

The superintendent also said during that interview that the park didn't support the alternative that would have phased out snowmobile traffic in favor of snowcoach-only access because park studies demonstrated "that there was (no) real difference between a group of snowmobiles and a snowcoach. They’re very comparable in terms of air quality, in terms of sound, in terms of wildlife harassment, and so the question was why would we?”

As for keeping Sylvan Pass open, Superintendent Wenk said his staff was of the opinion that it can continue to be operated safely.

Asked whether the cost of controlling avalanches along the pass, at $125,000 per winter, or roughly $1,200 per visitor via that route, was justified, he said that was a tough question.

“I don’t know. The (overall park) traffic doesn’t justify the expense for the winter. When I’m dealing with $60 a visitor in the winter, for access to the interior of the park, vs. $10 in the summer, does it justify the expense?" he said.

While Sylvan Pass might harbor wolverines -- tracks were found there in 2009 --, and the U.S. Fish and Wildlife Service recently proposed to list the carnivore as a threatened species, a listing that could affect the park's use of explosives to control avalanches on the pass, the final winter-use plan took a wait-and-see approach on how a listing might impact winter use of the pass.

Sylvan Pass, the document said, represents less than 0.1 percent of wolverine habitat, and the animals feed on winter-killed deer, bison, and elk, which are not present on the pass in winter.

"Procedurally, it is not feasible to examine impacts or manage for species that are not yet listed. Should wolverines come under the protection of the Endangered Species Act, the NPS will consult with the USFWS and, if necessary, make adjustments to the winter-use management framework," park planners said.