Official Proposal to extend US 14 across Idaho into Oregon (Oct 1936) -
See pg. 34 of the 5th pdf at https://drive.google.com/drive/folders/1kmrrq5P2bx3RQgkhu-JXDBWABUV85b_X
Would've followed US 91-191 to Idaho Falls then essentially today's US 20 to Oregon to meet US 28...
I've heard of a US 14 proposal to Nevada, but never a US 14 proposal to Oregon. I think Yellowstone National Park was a good terminus for US 14 (and 16).