PORTLAND, Ore. -- How many times does a road have to be blocked by a landslide before a barrier goes in to prevent it?

KGW took that question to The Oregon Department of Transportation and the Portland Bureau of Transportation, who both keep prioritized lists of the most dangerous spots.

They say when funding comes up, they look at what mitigation would work best in that specific area: laying fencing down the slope to prevent large rocks from falling, a permanent retaining wall like the ones along I-5 through the Terwilliger curves, or sometimes cutting into the slope to make it less steep and provide room between a hillside and the roadway.

Just last spring, PBOT put up rockslide fencing on Northwest Cornell Road, near the Audubon Society. But already, three landslides have occurred this winter in spots to the left and right of the fence.

Both agencies say they look at frequency and danger of landslides.

"We look at factors like geology and geometry of the slopes, frequency of landslides that occur in that location and we also look at traffic volumes and use of the highway, from there we'll prioritize sites around the region," said ODOT geotechnical engineer Tom Braibish.

Highway 99E near Canby is a notorious slide spot, and is number one to fix on ODOT's list. Next, is Highway 30 near the St. Johns Bridge, Highway 35 near Mount Hood and the the Jefferson Street on-ramp to Highway 26.