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18 ume of passenger-car traffic using the highways at such points. The over-all width of the open bands represents the total passenger-car traffic, and the width of the narrower black bands the volume of that part of the traffic, described as “foreign,” consisting of passenger cars registered in all States other than the State in which they were observed.

As it was the purpose of the map to show only the passenger-car movements of longer range, local increases of total traffic caused by short trips in and out of cities and local increases of “foreign” traffic caused by short trips over State lines are not shown. Shorn of these increases, the band widths, as shown, represent approximately the volume of all passenger-car traffic flowing, exclusive of the numerous extremely local movements mentioned. Thus, even the open bands represent traffic of relatively long range, and the black bands represent generally movements that are still longer and at least of interstate extent.

Examining the map, some of these existing routes are seen to stand out above others as relatively important in respect to both their total traffic and the volume of “foreign” traffic they serve; and it was by such a visual comparison, qualified by the various general considerations previously described, that certain routes were tentatively chosen as representing the approximate lines of the six superhighways to be investigated.

The routes thus chosen are indicated by the darker traffic bands on plate 8. On this map the width of the shaded bands represents the same total traffic that is indicated on plate 7 by the width of the open bands; and the generally superior importance of the selected routes is borne out by the generally greater width of the darker bands.

Tentative locations for three east-west and three north-south superhighways approximately paralleling the existing routes indicated in plate 8 were defined by the Bureau and submitted for consideration and criticism to the highway departments of each of the 48 States. In accordance with the comments received, several adjustments were made, and a final decision was reached, with the concurrence of all States except South Dakota, upon the routes as shown and numbered on plate 9.

For purposes of designation, the three north–south routes were numbered from the easternmost to the westernmost 1, 3, and 5; and the three east–west routes were numbered from the northernmost to the southernmost 2, 4, and 6. The diagonal route running northwestward from Salt Lake City was designated 4N (north) and the diagonal running southwestward from the same city, 4S (south). The branch of route 4 added to connect with Washington, D. C., was designated 4A; and because route 3 is broken where its location coincides with that of route 2, east of Chicago, the section extending northeastward from route 2 was designated as route 3 Mich.