If you're stuck in I-40 traffic some time in the next few months and you spot a bus rolling along the shoulder, it's probably not in trouble.

It's in an experiment.

The state Department of Transportation, Triangle Transit and several other members of an "I-40 Partnership" are going to try a "bus on shoulder system" on a section of Interstate 40 in southern Durham County. The I-40 Partnership is coordinated by the Regional Transportation Alliance, an association of Triangle chambers of commerce.

The system allows public-transit buses to use the shoulder as a regular lane of travel. The experiment will test whether the system can help alleviate I-40 congestion.

"It's the most heavily traveled and congested highway in the region," said city Transportation Director Mark Ahrendsen. "Over 140,000 cars a day in the Research Triangle Park and growing. It basically serves as the Triangle's Main Street."

Under the bus-on-shoulder system ("BOSS"), Triangle Transit buses may use the shoulder when traffic in the highway's regular lanes is moving at less than 35 miles per hour, said Meredith McDiarmid, DOT's corridor executive for Interstate 40 in the Triangle.

Buses do have to give way to vehicles stopped on the shoulder and emergency vehicles, she said.

"The first time I saw this I thought this was the craziest thing I ever heard," said City Councilman Mike Woodard. "But as I've been learning more about it ... I'm glad Durham's on the leading edge of good transit."

McDiarmid said the idea originated in Minnesota in the early 1990s. Eleven other states have since adopted it.

Triangle transportation planners and law-enforcement officers visited Minnesota to see the system in operation, she said; and, to try it out locally, rode a bus along an I-40 shoulder last week (with Highway Patrol escort).

"It was a very successful ride," I thought," McDiarmid said.

The bus-on-shoulder plan will begin within the next six months, she said. Its first sections will be westbound from N.C. 147 to U.S. 15-501 and eastbound from 15-501 to Page Road.

"It was not a conscious choice just to be in Durham County," McDiarmid said. The Transportation Alliance wanted a test segment without many obstructions, such as bridges, that would force buses back into regular travel lanes. The chosen section has wide shoulders all the way and no "pinch points," she said.

If the pilot program works out and nothing happens to make officials "uncomfortable," it will remain in effect permanently and be extended to other roads in the Triangle and other parts of the state.

"It's a great tool we can use to promote more bus transit ridership," said Woodard, but it's going to take a lot of educating to get the motoring public ready for it.

"If you're sitting on I-40 and all of a sudden here comes a bus on that right hand side of the road, that's a different thing for us," he said. "This is very, very different for us."