When first designated as such in January 1964, SR 240 began at Jct SR 24 at the West Hanford Gate and headed SE to Richland, ending at Jct US 410 at the Richland "Y". The highway across the Hanford Reservation did not actually open to traffic until the later 1960s. US 410 through Richland became US 12 in 1967. In the mid 1980s, I-82 was opened to traffic between Richland and Pasco. Part of SR 240 was realigned in the vicinity of I-182, joining I-182 at Exit 4 and leaving I-182 at Exit 5. When I-82 was completed on the SW side of the Tri-Cities and I-182 was extended west to I-82, US 12 was rerouted from Richland to Pasco via I-182. US 12 had gone via Kennewick between Richland and Pasco. SR 240 was extended from the Richland "Y" via former US 12 to Jct US 395 in Kennewick. SR 240 is now 41.34 miles long.
Before 1964, SR 240 from Jct SR 24 to Jct SR 224 in Richland had been SSH 11C and from SR 224 to US 410, part of SSH 3R.
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