The road over Sylvan Pass east of the east entrance of Yellowstone National Park reopened Friday evening after the discovery earlier that day of an unexploded device used for avalanche control, a park spokeswoman said.

"There was a little bit of a traffic build-up since we are still on the down-side (of the tourist season) but still on our summer visitation," Julena Campbell said Monday.

"But I think most folks would prefer to sit in traffic for a little while versus the alternative, which is driving past a live explosive," Campbell said.

Friday afternoon, a park maintenance employee was picking up trash on U.S. Highway 14/16/20, she said.

He pulled over to pick up a hubcap and saw the unexploded 75 mm recoilless rifle round 12 feet from the road. He notified park authorities, who shut down the road about 3:30 p.m. and called Malstrom Air Force Base near Great Falls, Mont., Campbell said.

The road was closed from a few miles east of the East Entrance of teh park to Eleanor Peak Trailhead as a "hard closure" for eight miles, meaning no one was allowed. From Eleanor Lake to Pelican Creek was a "soft closure" of about 18 miles because people were on the trails. Park staff didn't allow anyone into the area, but they didn't go find those who were.

Technicians from the base came to the site, placed plastic explosive around the device and blew it up there, she said. "It basically annihilates it so there's nothing left."

The road was reopened about 8 p.m., Campbell said.

The pass receives a lot of snow, and the park tries to keep the pass open during the winter, which requires extensive avalanche control. The hillside of avalanche control is permanently closed to hiking and other recreational activities.

Until about 20 years ago, park workers used the 75 mm rounds, which are slightly less than 3 inches in diameter and about 12 to 14 inches long, she said. The park then began using 105 mm rounds fired from howitzers.

Friday's incident marked only the third time since the mid-1990s when an unexploded round has been found, she said, citing a park ranger. The previous rounds were found in 2005 and 1998, she said.

Park workers look for any fragments of exploded rounds and especially look for unexploded ones on the hillside where avalanche control happens, Campbell said. "It's pretty rare that we find one mid-summer, or at the end of the summer like this."