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40 10,000 or more vehicles. The average traffic carried by all rural roads conforming closely to the system was 2,660 vehicles daily, and the total traffic movement, 78,208,300 vehicle-miles daily. The latter was 16.79 percent of the 465,753,000 daily vehicle-miles served by all rural roads in 1941. No similarly exact data are available to show the traffic served by existing city streets approximating the location of urban sections of the system, and were such facts available, they would be of little significance as a basis for an estimate of the traffic that would be served by more adequate facilities.

In estimating the probable traffic use of the recommended system, the committee has made due allowance for shifts of existing traffic flow that would be induced by a preferential improvement of the recommended routes. Its estimate is that the system, as it probably would be constructed, would represent only about 1 percent of the total mileage of rural roads and streets, but would serve at least 20 percent of the total vehicle-mileage generated on all roads and streets.

Location in relation to principal topographic features.—The location of the recommended routes has been influenced in remarkably few places solely by consideration of topography. A knowledge of the general topography of the country is nevertheless essential to a full appreciation of reasons for the varying sizes of interstices between the meshes of the system in different parts of the country and for the few places in which apparent indirection of the lines of the system would otherwise be unaccountable. The overlay of the recommended interregional routes on a photograph of a relief map of the United States, reproduced as figure 21, indicates clearly the effect of the conformation of the land and of the courses of principal rivers in influencing the location of the routes.

In selecting the routes to comprise the system and in determining the extent of the system to be recommended, the primary purpose was to select routes forming an integrated system of reasonably limited total extent which would join the principal centers of population and industry in each geographic region with centers of similar relative importance in other geographic regions, by lines as direct as practicable.

The principal determinants in this selection were, therefore, the interconnection of the larger cities in all regions, accommodation of short-run traffic in and about lesser centers insofar as practicable, and creation of a system of optimum extent and maximum utilization.

INTERCONNECTION OF LARGER CITIES

As proof of the importance of interconnecting the major cities, evidence is here presented which indicates that nearly 90 percent of the traffic moving on main highways has either or both its origin and destination in cities, that traffic steadily increases with increased proximity to cities, that on transcity connections of main routes traffic mounts to volumes far greater than the general levels on rural sections, and that the heavily traveled sections of the proposed interregional system lic mainly within relatively narrow zones of traffic influence about cities of 10,000 or more population.