However, some route consideration was given. The preferred route would pass through the Rocky Mountain Trench from Prince George, British Columbia to Dawson City before turning west to Fairbanks, Alaska.
The attack on Pearl Harbor and beginning of the Pacific Theatre in World War II, coupled with Japanese threats to the west coast of North America and the Aleutian Islands, changed the priorities for both nations. On February 6, 1942 the construction of the Alaska Highway was approved by the United States Army and the project received the authorization from the U.S. Congress and President Franklin D. Roosevelt to proceed five days later. Canada agreed to allow construction as long as the United States bore the full cost, and that the road and other facilities in Canada be turned over to Canadian authority after the war ended.
The official start of construction took place on March 8, 1942 after hundreds of pieces of construction equipment were moved on priority trains by the Northern Alberta Railways to the northeastern part of British Columbia near Mile 0 at Dawson Creek. Construction accelerated through the spring as the winter weather faded away and crews were able to work from both the northern and southern ends; they were spurred on after reports of the Japanese invasion of Kiska Island and Attu Island in the Aleutians. On September 24, 1942 crews from both directions met at Mile 588 at Contact Creek and the highway was dedicated on November 20, 1942 at Soldiers Summit.
The needs of war dictated the final route, intended to link the airfields of the Northwest Staging Route that conveyed lend-lease aircraft from the United States to the Soviet Union. Thus, the rather impractical, long route over extremely difficult terrain was chosen.
The road was originally built mostly by the US Army as a supply route during World War II. There were four main thrusts in building the route: southeast from Delta Junction, Alaska toward a linkup at Beaver Creek, Yukon; north then west from Dawson Creek (an advance group started from Fort Nelson, British Columbia after traveling on winter roads on frozen marshland from railway stations on the Northern Alberta Railways); both east and west from Whitehorse after being ferried in via the White Pass and Yukon Route railway. The U.S. Army commandeered equipment of all kinds, including local riverboats, railway locomotives, and housing originally meant for use in southern California.
Although it was completed on October 28, 1942 and its completion was celebrated at Soldier's Summit on November 21 (and broadcast by radio, the exact outdoor temperature censored due to wartime concerns), the "highway" was not usable by general vehicles until 1943. Even then, there were many steep grades, a poor surface, switchbacks to gain and descend hills, and few or no guardrails. Bridges, which progressed during 1942 from pontoon bridges to temporary log bridges, were replaced with steel bridges where necessary only. One old log bridge can still be seen at the Aishihik river crossing. The easing of the Japanese invasion threat resulted in no more contracts being given to private contractors for upgrading of specific sections.
In particular, some 100 miles of route between Burwash Landing and Koidern, Yukon, became virtually impassable in May and June of 1943, as the permafrost melted, no longer protected by a layer of delicate vegetation. A corduroy road was built to restore the route, and corduroy still underlays old sections of highway in the area. Modern construction methods do not allow the permafrost to melt, either by building a gravel berm on top or replacing the vegetation and soil immediately with gravel. However, the Burwash-Koidern section is still a problem, as the new highway built there in the late 1990s continues to experience frost heave.
The pioneer road completed in 1942 was approximately 1,680 miles from Dawson Creek to Delta Junction. The army then turned the road over to the Public Roads Administration of Washington, which then began putting out section contracts to private road contractors to upgrade selected sections of the road. These sections were upgraded, with removal of excess bends and steep grades; often, a traveler could identify upgraded sections by seeing the telephone line along the PRA-approved route alignment. When the Japanese invasion threat eased, the PRA stopped putting out new contracts. Upon hand-off to Canada in 1946, the route was 1,422 miles from Dawson Creek to Delta Junction.
The route follows a northwest then northward course from Dawson Creek to Fort Nelson. On October 16, 1957, a suspension bridge crossing the Peace River just south of Fort St. John collapsed. A new bridge was built a few years later. At Fort Nelson, the road turns west and crosses the Rocky Mountains, before resuming a westward course at Coal River. The highway crossed the Yukon-BC border nine times from Mile 590 to Mile 773, six of those crossings were from Mile 590 to Mile 596. After passing the south end of Kluane Lake, the highway follows a north-northwest course to the Alaska border, then northwest to the terminus at Delta Junction.
Postwar rebuilding has not shifted the highway more than ten miles from the original alignment, and in most cases, by less than three miles. It is not clear if it still crosses the Yukon-BC border six times from Mile 590 to Mile 596.
The Milepost, an extensive guide book to the Alaska Highway and other highways in Alaska and Northwest Canada, was first published in 1949 and continues to be published annually as the foremost guide to travelling the highway.
The British Columbia government owns the first 82.6 miles of the highway, the only portion paved during the late 1960s and 1970s. Public Works Canada manages the highway from Mile 82.6 (km 133) to Historic Mile 630. The Yukon government owns the highway from Historic Mile 630 to Historic Mile 1016 (from near Watson Lake to Haines Junction), and manages the remainder to the U.S. border at Historic Mile 1221. The State of Alaska owns the highway within that state (Mile 1221 to Mile 1422).
Extensive rerouting in Canada has shortened the highway by approximately 35 miles (55 km) since 1947, mostly by eliminating winding sections and sometimes by bypassing residential areas. Therefore, the historic milepost markings are no longer accurate but are still important locally as location references. Some old sections of the highway are still in use as local roads, while others are left to deteriorate and still others are ploughed up. Four sections form local residential streets in Whitehorse (3... see map) and Fort Nelson (1), and others form country residential roadways outside of Whitehorse. Although Champagne, Yukon was bypassed in 2002, the old highway is still completely in service for that community until a new direct access road is built.
Rerouting continues, expected to continue in the Yukon through 2009, with the Haines Junction-Beaver Creek section covered by the Canada-U.S. Shakwak Agreement. The new Donjek River bridge was opened 26 September 2007, replacing a 1952 bridge. Under Shakwak, U.S. federal highway money is spent for work done by Canadian contractors who win tenders issued by the Yukon government. The Shakwak Project completed the Haines Highway upgrades in the 1980s between Haines Junction and the Alaska Panhandle, then funding was stalled by Congress for several years.
The Milepost shows the Canadian section of the highway now to be approximately 1187 miles, but the first milepost inside Alaska is 1222. The actual length of the highway inside Alaska is no longer clear because rerouting, as in Canada, has shortened the route, but unlike Canada, mileposts in Alaska are not recalibrated. The B.C. and Yukon governments and Public Works Canada have recalibrated kilometreposts only as far as a point just at the southeast shore of Kluane Lake, with the latest BC recalibration in 1990 and the only Yukon recalibrations in 2002 and 2005 (based on the distance value where the BC calibration of 1990 left off).
There are historical mileposts along the B.C. and Yukon sections of the highway, installed in 1992, that note 83 specific locations, although the posts no longer represent accurate driving distance.
For people interested in learning more about the history of the Alaska Highway there are several books on its construction, including "Alcan Trail Blazers: Alaska Highway's Forgotten Heroes."
The Canadian section of the road was delineated with mileposts, based on the road as it was in 1947, until 1978, and over the years, reconstruction steadily shortened the distance between some of those mileposts. That year, metric signs were placed on the highway, and the mileposts were replaced with kilometre posts at the approximate locations of a historic mileage of equal value, e.g. Kmpost 1000 was posted approximately where historical Mile 621 would have been posted.
Reconstruction continues to shorten the highway, but the kilometre posts, at two-km intervals, were recalibrated along the B.C. section of road in 1990 to reflect then-current driving distance. The section of highway covered by the 1990 recalibration has since been rendered shorter by further realignments, such as near Summit Pass and between Muncho Lake and Iron Creek.
Based on where those values left off, new Yukon kilometre posts were erected in fall 2002 between the B.C. border and the west end of the new bypass around Champagne, Yukon; in 2005, additional recalibrated posts continued from there to the east shore of Kluane Lake near Silver City. Old kilometre posts, based on the historic miles, remain on the highway from that point around Kluane Lake to the Alaska border. The B.C. and Yukon sections also have a small number of historic mileposts, printed on oval-shaped signs, at locations of historic significance; these special signs were erected in 1992 on the occasion of the highway's 50th anniversary.
The Alaska portion of the highway is still marked by mileposts at one-mile intervals, although they no longer represent accurate driving distance, due to reconstruction.
The historic mileposts are still used by residents and businesses along the highway to refer to their location, and in some cases are also used as postal addresses.
Residents and travellers, and the government of the Yukon, do not use "east" and "west" to refer to direction of travel on the Yukon section, even though this is the predominant bearing of the Yukon portion of the highway; "north" and "south" are used, referring to the south (Dawson Creek) and north (Delta Junction) termini of the highway. This is an important consideration for travellers who may otherwise be confused, particularly when a westbound travel routes southwestward or even due south to circumvent a natural obstacle such as Kluane Lake.
Some B.C. sections west of Fort Nelson also route more east-to-west, with southwest bearings in some section; again, "north" is used in preference to "west".
Other roads that join the Alaska Highway include, from South to North:
(the Cassiar highway is mostly within B.C., joining the Alaska Hwy one mile inside the Yukon)
Fort Nelson - Mile 301 to 308, now local residential feeder roads Wildflower Drive, Highland Road, Valleyview Drive
Champagne-Aishihik traditional territory
Other former segments have deteriorated and are no longer usable. More recent construction projects have deliberately ploughed up roadway to close it.