Two different state highways have been designated as SR 170 at different times since the current state highway numbering system was first posted in 1964.
The earlier SR 170
The earlier SR 170 followed part of what had been SSH 11A. Before WWII, SSH 11A started at Jct PSH 11 (US 395) in Connell and headed west, crossed the Columbia River on a ferry from White Bluffs to Hanford,and continued west to Jct PSH 3 (US 97/US410) in Yakima. During the war, the Hanford Site was created and SSH 11A across the site was closed to the public from the White Bluffs ferry landing to the vicinity of Cold Creek. In 1961, legislation added a Cold Creek to Othello segment to SSH 11A with a provision that the Cold Creek to Mesa vicinity portion of SSH 11A would be dropped from the state highway system upon completion of SSH 11C from SSH 11A near Cold Creek to PSH 3 (US 410) in Richland.
In January 1964, SSH 11A from Yakima via Cold Creek to Jct SR 26 in Othello became SR 24. At first, SSH 11A/SR 24 from Cold Creek to Othello crossed the Columbia River on a ferry near Vernita. A bridge replaced the ferry circa 1965. For some reason, the White Bluffs to Jct SR 17 in the Mesa vicinity portion of SSH 11A was not assigned an SR number in 1964. The SR 17 to Connell portion of SSH 11A was posted as SR 260. In 1965, the White Bluffs to SR 17 portion of SSH 11A became posted as SR 170. This route was dropped from the state highway system in 1967.
The later SR 170
During the same legislative session that the earlier SR 170 was dropped from the state highway system, a new SR 170 was added. This SR 170 begins at Jct SR 17 and heads east to Warden. Today, SR 170 is 3.66 miles long.
From 1967 to 1970, the unposted legislative route number for the later SR 170 was SSH 11I.
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