be added to the System within the authorized mileage ridors selected for the System. As more precise data became available, the mileage and location of the designated segments were adjusted accordingly.
Savings in the total System length, resulting from more detailed studies of the related engineering, economic and sociological factors involved in the highway location, permitted added System segments under the original statutory length limitation. In addition, Congress in the Federal-Aid Highway Act of 1956 increased the authorized length of the Interstate System by 1,000 miles. Thus, the Secretary of Commerce, on October 18, 1957, was able to announce an increase of 2,102 miles of Interstate routes that could limit. With these additions, there remained a reserve of 350 miles which was held to cover possible increases in the length of individual route segments as more detailed location studies progressed.
Section 17 of the Hawaii Omnibus Act of 1960 (PL 86-624) included an amendment that removed the limitation that the Interstate System be designated entirely within the continental United States. Under the provisions of this amendment, three routes on the island of Oahu in the State of Hawaii were approved as additions to the System in August 1960, utilizing some of the “reserve” mileage noted above. And in the Federal-Aid Highway Act of 1968, Congress authorized an additional 1,500 miles on the Interstate System. This led to an announcement in December 1968 of the designation of route segments totaling 1,473 miles in length.
Subsequent System adjustments, additions and deletions have resulted in the present status of full commitment of the total 42,500 miles authorized by Congress for the Interstate highways.
The rest and information area on I-90 near Sheridan, Wyo., provides picnic areas for the family and play areas for children. A choice of a moment beside the delicate ponds or a lovely view of mountains is also available to arrest the traveler’s eye.
Signing and Numbering
From the very nature of Interstate System traffic, signs on the System need to be highly visible, properly located, clearly understood, and completely uniform. After study of existing practices on expressways and field tests by the Bureau of Public Roads, the American Association of State Highway Officials in 1958 adopted a manual on signing and pavement marking for the Interstate System. The manual prescribed as the Interstate route number marker the now familiar red-white-and-blue shield. Guide signs, primarily for carrying destination information, are uniformly white on green; signs concerning services and rest areas are white on blue. The manual also specified the message sizes and, in general terms, the placement of signs so that uniformity will prevail throughout the System.
The American Association of State Highway Officials also developed a complete numbering system for Interstate routes. Those routes with odd numbers run north–south; those with even numbers run east–west. Major routes have one- or two-digit numbers, and the long, evenly spaced routes have numbers ending in 5 or 0. The lowest numbers are in the west and south to avoid conflict locally with the U.S. Route numbers. In urban areas the main route numbers are carried through on the paths of the major traffic streams. Connecting circumferential or loop routes at urban areas have three-digit numbers, using the main route number with an even-number prefix. Radial and spur routes also have three-digit numbers, with an odd-number prefix.
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