Wikipedia:WikiProject U.S. Roads/Idaho/early history

Initial routes

Good Roads, June 5, 1915, p. 219: The Survey of the Idaho State Highway System [1] (this is kind of screwed up, probably due to a printing error)

An account of the work of surveying the roads comprised in the state highway system of Idaho is given in the first biennial report of the State Highway Commission of that state for the period ending December 31, 1914.

The State Highway Commission was established in 1913 by a law which authorized the providing of a trunk line system to be built by contract under the supervision of the state. It was provided that the cost of construction should be borne jointly by the state and the counties through which the roads ran, the state paying one-third and the counties two-thirds, and that the roads should be maintained by the state. The first work undertaken by the commission was that of selecting the routes of the state highways. They are as follows:

The Idaho-Pacific Highway, commencing at the Idaho-Utah line near the southeastern corner of Idaho and extending northwesterly, then westerly, then again northwesterly through Montpelier, McCammon, Pocatello, American Falls, Burley, Twin Falls, Buhl, Hagerman, Bliss, Mountain Home, Boise, Nampa, Caldwell and New Plymouth to Payette, thence northerly near the western border of the state, through Weiser, Council, Pollock, Grangeville, Nez Perce, Lewiston and Moscow to the Idaho-Washington line, near Moscow, a total distance of about 800 miles. It is expected to extend this survey later to Sandpoint via Coeur d'Alene, an additional distance of about 150 miles.
The Idaho-Montana Highway, extending from a point on the Idaho-Montana line near Gibbonsville in Lemhi County and extending southeasterly through Gibbonsville, Salmon, Leadore, Gilmore and Roberts to Idaho Falls, thence southwesterly through Blackfoot to a connection with the Idaho-Pacific Highway at Pocatello, a total distance of about 200 miles.
[?] Idaho-Utah line in Franklin County, near the southeastern corner of the state, and extending in a general northerly direction through Preston, Dayton and Downey to a connection with the Idaho-Pacific Highway, 2 miles south of McCammon, a total distance of about 57 miles.
The Yellowstone Park Highway, commencing at the Targhee National Forest Reserve in the northeastern part of the state near Yellowstone National Park and extending southeasterly through Bellevue to Tikura and thence south- [?] Rigby to a connection with the Idaho-Montana Highway at Idaho Falls, a total distance of about 66 miles.
The Sawtooth Park Highway, extending from Hailey in Blaine County, in the south central portion of the state, southwesterly through Ashton, St. Anthony, Rexburg and westerly through Richfield, Shoshone and Jerome to a connection with the Idaho-Pacific Highway near Twin Falls, a total distance of about 100 miles.
The North Pacific Highway, commencing at the Idaho-Montana line at St. Regis Pass in the northern portion of the state and extending northwesterly across the state through Mullan, Wallace and Cataldo to Coeur d'Alene and a connection with the "Apple Way," a road connecting Coeur d'Alene and Spokane, Wash., a total distance of about 82 miles.

As estimates on about a thousand miles of the highways included in these routes were desired as soon as possible, it was decided to make the surveys without...

Motor West, November 1, 1916, p. 21 [2]

This system of trunk lines consisted of the Idaho-Pacific Highway, which traverses the southern portion of the state east and west from Utah to Oregon, the North and South Highway from Weiser to Moscow, the North Pacific Highway across the Panhandle from St. Regis Pass to Spokane Bridge, the Yellowstone Park Highway from Pocatello to Yellowstone, the Sawtooth Park Highway from Twin Falls to Hailey and the Idaho-Utah Highway from McCammon south to the Utah line near Preston.