bear jam
n. A traffic jam in a park caused by motorists stopping to watch one or more bears.
Yellowstone averages roughly three million visitors per summer (about 30,000 per day, especially in July and August). Along with the road projects, the "bear jams" (this term applies to other wildlife such as bison and elk) cause lines of vehicles to clog the roads as tourists stop to photograph wildlife.
—Bill Croke, “Disney's Last Frontier,” The American Spectator, May 06, 2005
Roadside bears that prompt cars to stop, causing traffic jams, have become the park's biggest bear problem, according to park managers.

Superintendent Suzanne Lewis told a group of bear managers in Jackson recently that policing bear jams has become Yellowstone's top priority.

Tourists are not always helpful in such situations. One biologist recently told of a tourist who swatted a grizzly cub on the rump, prompting the mother bear to charge. …

Some 700 bear jams were recorded last year and probably not all jams were recorded, said Kerry Gunther, park bear management biologist.
—“Roadside bears the biggest bear problem in Yellowstone,” The Associated Press, April 11, 2003
1983 (earliest)
''Bear jams,'' those terrific traffic tie-ups that made Yellow Stone National Park famous worldwide, are fast becoming a thing of the past, CBS News reports tonight in a saddening hour-long special, ''Paradise Lost.''

Only 20 years ago photo-hungry motorists trolling the roadways of the world's first national park were bound to see packs of bears frolicking on the dusty shoulders.

Now, millions of them go home without seeing any bears, park rangers tell CBS News, especially grizzly bears.
—Julianne Hastings, “TV World: Grizzly bears' days numbered, CBS special warns,” United Press International, September 02, 1983