Congress designated 40,000 miles of highways as Interstate highway corridors in 1944 but did not appropriate any money to build the Interstate system. The 1956 federal aid highway bill funded the construction of the Interstate system and added 1,000 miles of highway to the system. The specific routes to be added were not spelled out by the 1956 bill, however.
In addition to the 1,000 miles added to the Interstate system by law in 1956, another 1,000 miles were freed up for reallocation by shortening the alignment of Interstate corridors through taking shortcuts not followed by the highways, mostly US numbered routes, that had been designated as Interstate in 1944. Highway News, the magazine of the Washington State Department of Highways, states in its November-December 1957 issue that Washington and Oregon were to get a new Interstate route to link the Ellensburg and Pendleton areas. The article mentions that the the route is to be called Interstate 82 and tht the I-5 and I-90 numbers were to be assigned to the US 99 and US 10 Interstate corridors. The article does not spell out the precise route of I-82.
An article in the May-June 1958 issue of Highway News. including a map, talks about a change to the route. Earlier plans had included a new bridge across the Columbia River near Paterson. The new plans called for crossing the Columbia at Umatilla, the route ultimately built. The map shows the highway from Yakima to the Columbia heading down the Yakima Valley to Satus, then over the Horse Heaven Hills to Paterson, then east to Plymouth and the Plymouth-Umatilla bridge. The route from Yakima to the Columbia ended up being built along a different route than shown on this map.
Further modifications to plans for the route of the Yakima-Plymouth segment of I-82 were made. One plan called for following the Yakima Valley to Mabton, then crossing Hores Heaven Hills. Another called for traveling down the valley to Prosser, then crossing the Horse Heaven Hills. By the early 1970s, I-182 had been designated as a spur to connect I-82 in the lower Yakima Valley with the Tri-Cities. In fact, the Benton City-Kiona Interchange, I-82 Exit 96, was built in the early-mid 1970s with money allocated to I-182.
By the mid 1970s the state had settled on the route that was ultimately built. This route avoids the Yakama Reservation, except for a short section just south of Union Gap, staying close to the route of former US 410 to the western outskirts of the Tri-Cities. I-182 begins at this point and heads east to Richland and Pasco. I-82 heads southeasterly to the south side of the Tri-Cities and then southerly to the Columbia River and Oregon.
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