LEWISTON, Idaho—A Canadian oil company's $8 billion plan to tap Alberta's oil sands has hit a roadblock en route to U.S. Highway 12.

Imperial Oil wants to send 35,000 tons of mining equipment over the Rocky Mountains in 207 massive loads that would creep along the winding, two-lane highway in the middle of the night—four or five shipments a week—for about a year. The route is part of a shortcut of sorts, designed to shave months and thousands of miles off the trip from the equipment's South Korean maker to Canada.

For now, though, the equipment modules are starting to pile up at the Port of Lewiston, waiting for an Idaho Supreme Court ruling that could keep the plan in park.

Imperial Oil, a Canadian company working with Exxon Mobil Corp. on the project, says it has spent years hammering out deals with transportation officials both here and in Montana, and will spend about $100 million in the two states to ensure its equipment arrives safely. But some residents want to keep the industrial cargo off the scenic highway, worried both about planned road closures and what would happen in case of an accident.