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The System of Interstate Highways
The routes recommended by the Joint Board and confirmed by the several States are shown on the map accompanying this report.
So far as possible the routes selected have adhered to the federal aid highway systems already approved for the several States. Practically all of the States, however, have some small margin within the legal limit of seven per cent which comprises the full federal aid system, and this margin has allowed minor departures from the approved federal aid systems. These new sections, as may be required, can at the request of the respective States, "be added to the federal aid systems and in some instances States have already filed applications for such additions.
As already indicated by the Mileage Tables, it was obviously necessary to exceed in some States the three per cent prescribed by law for a primary system of roads. To have arbitrarily adhered to the three per cent limit in several Western States would have resulted in omitting many desirable and needed routes. In the country as a whole, however, the total mileage of routes selected by the Joint Board is 2.6 per cent of the total certified public road mileage and, therefore, is within the three per cent primary classification permitted by law.
Having selected a system of routes for uniform marking, the question of designation was considered and an effort made with gratifying success to introduce an orderly arrangement of routes. In general, the scheme involves the use of even numbers for routes carrying east and west bound traffic, and odd numbers for the north and south routes. An unbroken numerical sequence was not possible unless lines of prevailing flow of traffic were to be entirely neglected. Such lines cross each other and demand that numerical order be sacrificed in a few cases. These are, however, so few and slight that the value of the numbering scheme is not diminished for practical purposes. The routes given continuous designations have been carefully considered and so far as possible are those (1) which are carrying on the whole the heaviest long distance traffic, (2) which the States contemplate improving to high standards, and (3) which are in the construction program for early improvement.
Conclusion
The Joint Board has included among its recommendations that the Secretary of Agriculture refer this report to the several State highway departments and to the Association of State Highway Officials. This recommendation is made in order to accomplish the practical application of the work of the Board both as respects the designation and marking of interstate routes and the adoption and erection of uniform traffic signs. It is known to the Board that several States are now awaiting
