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98 totaling 2,488 miles or 8.5 percent of the total rural mileage were paved with bituminous concrete or sheet asphalt pavements, and 14,602 miles or 49.6 percent of the total were paved with concrete, brick, block, or some combination of high-type pavements.

In general design at least, there is a marked correlation between the surfaces and pavements of the existing roads and the volume and weight of the traffic they serve. Grouping untreated gravel and stone surtaces as low types; bituminous surface-treated gravel and stone, mixed bituminous surfaces, and bituminous penetration surfaces as intermediate types; and bituminous concrete, concrete, brick, and block as high types, the sections of the system improved with each of these classes of surfaces are indicated by type symbols in the map, figure 32. Comparison of this map with the traffic map presented as figure 20 will confirm the statement that there is a strong correlation between the existing surface types and traffic volume.

Adequacy of design.—But while the existing roads may be said to be reasonably well improved so far as the character and strength of their surfaces are concerned, they are far from adequate in respect to those characteristics of their design that have a bearing upon their ability to discharge their traffic without congestion. These characteristics are the width and lane arrangement of the surfaces or pavements, gradients and curvature, and the related characteristic of sight distance.

To obtain the additional width and lane arrangement required for conformity to the recommended standards will necessitate almost universal widening.

Here we encounter the deplorable fact that existing rights-of-way are grossly insufficient to permit such widening.

Even, therefore, if it were possible to attain the recommended standards of design without change of existing alinement, a right-of-way problem of great difficulty would be presented, and the fact is that the faults of curvature and gradient are so numerous that no approximate compliance with the proposed standards can be achieved on most sections without wide departure from the existing alinement.

Taken together, the two circumstances of insufficient width and inadequate alinement, if the proposed standards are to govern, leave little choice in most sections of the system other than the obtainment of entirely new right-of-way; and this conclusion, reached from the consideration of essential dynamic qualities of the highways, agrees with the decision that must inevitably result from any consideration of a desirable directness of routing between the principal sources and objectives of interregional highway traffic.

Bearing out the foregoing general statements, figure 33 presents a graphical analysis of the average physical conditions of existing rural roads conforming approximately in location to routes of the recommended system, classified according to the average daily volume of traffic. From this figure it will be seen that the most lightly traveled roads conforming to the system are those that approach most nearly the standards proposed.