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48 The Committee reasoned that somewhere between these two extremes, employing basically the principle of the interconnection of larger cities, it should be possible to select a system of optimum extent, the average usage of which would reach a maximum of intensity. Considered as a whole, the average daily traffic volume for such a system would be greater than that for any other system either more or less extensive.

The Committee determined to select a system approaching as nearly as practicable this optimum extent. This it conceived it could do by selecting a number of systems, both larger and smaller than the probable optimum, and by plotting the average daily traffic of each against its extent in miles. In such a manner a curve would be formed, the maximum ordinate of which, representing the maximum daily traffic volume, would correspond to an abscissa representing the extent of the optimum system.

Data for such an analysis were available to the Committee in several studies previously made by the Public Roads Administration. One of these studies was that relating to the toll road system of 14,300 miles described in the report entitled “Toll Roads and Free Roads,” transmitted to Congress by the President in 1939. This system was regarded as very close to a system of minimum extent, and therefore probably below the optimum. Another was the 26,700-mile system described by the Public Roads Administration in the same report. Still another was a slight enlargement of the latter system, totaling 29,300 miles, which has been previously described in an article published in the magazine Public Roads. A fourth was a 48,300-mile system, and the fifth and last a system totaling 78,800 miles in extent. In these five systems the most important routes are substantially identical in location. The differences in total mileage result largely from the progressive addition of routes. Each is shown on a separate map in appendix II, figures 1 to 5, inclusive.

With respect to city connection, the extremes of these systems range from the smallest which omits direct connection between a number of cities of more than 300,000 population and one of 500,000 or more population, to the largest which connects directly a large percentage of all cities with population of 10,000 or more persons.

From data obtained by the highway planning surveys, the total traffic service of existing rural roads conforming closely to each of these five previously investigated systems was estimated in daily vehicle-miles, and the corresponding average daily traffic volumes were computed. These data, together with the mileages of the systems, are given in the upper section of table 13.

From this table the values for the mileage and average daily traffic of each of the five systems were taken and plotted as points on a system of rectangular coordinates, as indicated by the outline dots in figure 24. These points were then connected in various ways by straight lines.