1964 state highway renumbering (Washington)

The former numbering system of primary and secondary state highways, using lettered suffixes and unnamed branches, created confusion for motorists as the system expanded. The system also ignored, or conflicted with, the federal highway system and the then-developing Interstate Highway System.[1] The state highway department originally planned for a major highway renumbering in 1957, expanding on the existing primary and secondary system with numbers as high as 59, but serious consideration of a full-scale renumbering began in 1962.[2] It had the specific goal of replacing letter suffixes with two- and three-digit numbers, which would not repeat or conflict with each other or with federal route numbers.[3]

In 1963, the Washington State Legislature passed a law authorizing the creation of a new state highway numbering system under the direction of the Washington State Highway Commission.[4] The law came in response to confusion experienced by tourists visiting during the 1962 World's Fair, held in Seattle.[5] The highway commission approved its numbering plan on June 19, 1963, using even numbers for east–west routes and odd numbers for north–south routes; primary routes would have lower numbers, while secondary routes would use the first digit to indicate its parent route.[6][7] Federal highways, including Interstate highways, would retain their designations and be incorporated into the system.[8] The new numbering system took effect on January 15, 1964,[9][10] and new signs were posted at a cost of $115,000.[5][11]

• Washington State Highway Commission, Identification of State Highways Archived July 24, 2008, at the Wayback Machine, December 1, 1965

• Highways of Washington State