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Rh In addition, however, to these defective bridges which should be rebuilt or altered, a minimum acceptable improvement of the rural network also requires the building of a number of other structures for various reasons. These include grade-separation structures required at existing grade intersections, bridges to replace existing fords and dips, and new bridges necessitated by desirable changes in the location of existing roads. Including these additional structures, the total number of bridges and underpasses involved in a minimum acceptable improvement of the rural network is 3,648.

Conditions of roads.—Of the 66,869 miles of existing rural roads conforming to main lines of the strategic network, 18,613 miles serve a normal traffic averaging less than 600 vehicles per day.

Of this latter mileage, roads included in the proposed interregional system make up 3,825 miles, or approximately 21 percent. The remaining 14,788 miles consist of roads not included in the interregional system.

Roads serving average daily traffic ranging between 600 and 1,800 vehicles total 29,273 miles. Of this, 11,750 miles, or approximately 40 percent, are included in the interregional system.

Roads serving traffic averaging 1,800 vehicles per day and more total 18,983 miles. Nearly 47 percent of this—9,979 miles—is in the interregional system.

These traffic-volume classes are associated with the standards of width for new work previously defined. These standards call for pavements not less than 20 feet wide where average daily traffic is less than 600 vehicles, not less than 22 feet where traffic averages between 600 and 1,800 vehicles per day, and not less than 24 feet for daily traffic averaging 1,800 vehicles or more.

In tables 5 and 6 the entire rural mileage conforming to the network is classified into traffic-volume and pavement-width groups. The tables indicate the extent to which existing pavements conform to the prescribed standards.