on monday (11/20/060) the national park service announced its intention to continue  perrmiting use of yellowstone national park by as many as 720 snowmobiles per day. 

nearly four years of ongoing intensive studies of snowmobile impact on yellowstone's ecosystem by the agency's own scientists have so far cost us $8 million dollars and generated three reports, all of which have concluded snowmobiles produce unacceptable levels of air pollution and noise.   other federal agencies--including the epa--agree and have supported previously proposed park service restrictions on snowmobile use inside yellowstone's boundaries. 

according to an article published in the 11/21/06 edition of the la times:

"though the park now requires snowmobiles to utilize a newly designed four-stroke engine, the technology has not restored the standard for natural quiet required by park service regulations. monitoring over three years has shown that the machines have been exceeding noise standards both for decibels and for the percentage of time the machines are heard throughout the park.

according to park service studies, the cleaner and quieter snow machines can be heard in about twice as much of the park as snow coaches.

in past winters, rangers at entrance stations have been advised by park officials to wear hearing protection."

Link

the article also quotes  senator craig thomas (r-wy)--who chairs the senate's subcommittee on national parks commenting on the park service plan:

"as with any plan that considers competing points of view, it's not perfect, but i'm pleased that our public land managers recognize the importance of maintaining visitor access to our national parks in wyoming."

perhaps  it's time to widen the scope of all this research to determine how many more studies and reports will it take before someone wises up and sufficiently tweaks the data or parameters to produce more acceptable results?