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able. All but 1,625 of these sections are on either two- or three-lane roads. The 1,625 sections on multilane highways total 161 miles in length; the two- and three-lane mileage involved is 1,926.

Safe and satisfactory operation on these sections can, in some cases, be provided by the removal of view obstructions such as trees, cut-slopes, and nearby buildings; but in the great majority the alinement or profile of the road, or both, must be changed to flatten horizontal or vertical curves.

Photo by Oregon State Highway Commission

The two-lane U S Route 99 in Oregon, carrying 2,150 vehicles a day, crossed this mountain pass with grades of 6 percent on each side of the summit. Tremendous excavation would have been required to provide adequate passing sight distance so the State widened the section over the summit to four lanes instead.

Safe passing distance and essential capacity

Passing sight distance is required only on two-way roadways of two and three lanes. Of these categories, on the rural system, there are 29,287 miles, including about 11 miles that are unsurfaced; and of this total, passing sight distance is undesirably restricted on 7,324 miles or about 25 percent.