An estimated 40 vehicles with flat tires were stranded mid-day Thursday on the eastbound 91 freeway at the border of Orange and Riverside counties after running over nails.

The 11 a.m. incident left vehicles stranded over a 15-mile stretch, between the Weir Canyon Road and McKinley Street exits. It took four hours to completely clear the road of cars and nails.

“We believe it was some sort of construction vehicle that was spilling them,” said Florentino Olivera, an officer and spokesman for the California Highway Patrol.

CHP officers slowed traffic on the freeway’s eastbound lanes while crews removed the 2-1/2-inch nails.

“It was hard, because they were embedded into the roadway, almost like spike strips,” Olivera said. “We got a sweeper out there and cleaned up the nails. … It was a mess.”

The eastbound side of the 91 Express Lanes, a tollway in the freeway’s median, was closed at one point.

The source of the spill had not been found.