But loose rocks and boulders that are sitting on the steep slopes, especially during a summer monsoon or a winter rainstorm, can pose a danger, unseen until it's too late.
"You can have a three-inch rock that flies through a window, and kill (someone) instantly if it has enough momentum," Caltrans geotechnical engineer Richard Rusnak said.
That's why the highway will be closed for the rest of the week. More than 20 Caltrans climbers are scaling the steep hillside more than 100 feet above the highway and knocking loose rocks and boulders. Early Tuesday morning, one boulder that was more than six feet in diameter came crashing down onto the closed highway.
Crew members rappel down the hillside from above, with ropes tied securely to trees. They make sure to shout warnings to each other whenever jarring a large boulder loose, just to be safe. Then, about every 20 minutes, a Caltrans snowplow comes through to move the rocks and boulders down the highway and off the side of the shoulder.
Still, it's a dangerous job. A few years ago, several Caltrans climbers in Northern California paid for it with their lives.
"They were working on a slope, and took out a rock and it started a landslide," Rusnak said. "They were underneath, and unfortunately, we lost them."
The operation runs through Friday. Highway 18 is closed from Snow Valley to the Big Bear Dam from 9 a.m. until 3 p.m. to allow the crews to work above the highway.