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108 An approximate evaluation of the minimum rate at which the rural sections of the system should be constructed can be made by determining the probable economic life remaining in the surfaces and pavements of existing highways conforming approximately in location to routes of the system.

Figure 35 shows, by the differential segments of the 1942 ordinate, the mileage classification of the various types of surfaces and pavements existing on highways conforming approximately to routes of the recommended system on January 1, 1942. The width of the shaded band projected from each of these Segments shows by the intercept at each subsequent annual ordinate the approximate mileage of the several types of surfaces and pavements existing on January 1, 1942, that will remain in economic life on January 1 of each of the several years. The open upper portion of the diagram shows similarly the 1941 traffic density classification of the mileage on which surface or pavement reconstruction will have become necessary by January 1 of each year.

The mileage of surface or pavement reconstruction, indicated by the depth of the open section of the diagram (according to the scale at the right) as likely to become necessary by January 1 of each year, includes only the first reconstruction of the surfaces or pavements existing on January 1, 1942. During the period of 20 years covered by the diagram, it is probable that some of the less durable surfaces constructed in replacement of existing surfaces will themselves depreciate to the point of essential reconstruction. This will depend upon the durability of surfaces and pavements constructed on the interregional system, and the amount of secondary reconstruction that will become necessary during the 20-year period will be reduced to a minimum by a policy of liberality in the selection of surface types for interregional system improvements.

If, in any period, as during the present war emergency, reconstruction becoming due is deferred, the amount essential will accumulate, and when the opportunity occurs it will be necessary to provide for an enlarged program of reconstruction to be continued for a period sufficient to meet the accumulated need.

For example, very little of the reconstruction indicated as of probable necessity by January 1, 1944, is now being accomplished.

Accordingly, as the diagram shows, there will have accumulated by the beginning of 1944 a need for the reconstruction of surfaces or pavements on about 1,700 miles of highways conforming approximately in location to routes of the recommended system; and this mileage will be distributed, 350 miles on sections which in 1941 carried a daily average of 3,000 but less than 10,000 vehicles, 300 miles on sections that carried between 2,000 and 3,000 vehicles, 500 miles on sections that carried between 1,000 and 2,000 vehicles, and 550 miles that carried in 1941 a daily traffic averaging less than 1,000 vehicles.

If the period of deferment should extend to January 1, 1945 or 1946, the total accumulated reconstruction need might be increased to 2,650 or more than 3,500 miles, respectively.

These estimated accumulated reconstruction needs are for rural sections only. They represent needs accumulated by the obsolescence of surfaces and pavements only and take no account of the greater needs of reconstruction to provide for safe and unobstructed traffic flow. They must be regarded, therefore, as minimum needs.