They rely on remote sensing technology to anticipate bad weather and keep the pavement clear. They work to unkink twists and turns left over from the original construction, when the builders dealt with muskeg and other obstacles by curving the road around them.

But today the Alcan faces challenges that could not have been predicted when it was built. By far the biggest is permafrost, the permanently frozen ground that underlies much of the road.

As the climate warms, stretches of permafrost are no longer permanent. They are melting — leaving pavement with cracks, turning asphalt into washboard and otherwise threatening the stability of the road.

Not all of the melting is due to climate change. Road improvements like heat-absorbing dark pavement alter conditions in the ground beneath, particularly if a lens of ice lies close to the surface. Merely removing roadside vegetation to uncover dark soil can have a melting effect.

Another problem is fire. “Even a natural forest fire will change the surface of the road,” leading to melting, said Bronwyn Benkert, who studies cold-climate issues at the Yukon Research Center, and who is researching highway conditions north of here, near the Alaska border.

But climate change is most worrisome of all. Not only is the world warming: it is warming fastest in high northern latitudes. And the problem is getting worse, with no easy solutions.

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If the permafrost is patchy — “discontinuous,” in geological parlance — even identifying areas of melt risk is tricky. Highway engineers have been drilling core samples along the roadway for 50 years, but “if you don’t drill in the right place you won’t find it,” Mr. Hidinger said. “We don’t even have a precise picture of the soil conditions under the road.”

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So engineers are using ground-penetrating radar and instruments to measure gravity variations, among other techniques, to try to identify permafrost areas.

In a test section near Beaver Creek, at the Alaska border, researchers and engineers are also experimenting with ways to limit permafrost melting along the highway, including rock embankments that insulate the ground in summer, keeping it relatively cool, and help carry cold air below ground in winter. Elsewhere, the engineers have replaced asphalt with light-colored pavement to reduce the amount of heat the road absorbs. Unfortunately, it costs up to 10 times as much as ordinary pavement.

The permafrost problem is not limited to the Alcan, of course. “Understanding permafrost is a necessary evil building roads in the north,” said Stephen Mooney, who directs the cold climate innovation project at the Yukon Research Center here. He hopes to encourage the use of standard terminology and measuring techniques so engineers will be able to learn from one another.

The Canadian government, as well as scientists in Alaska, Russia and other cold places are supporting research on permafrost — support that would have meant the world to the men who carved the pioneer road out of the forest.

A Strategic Corridor

They began with only a vague idea of where it should run and hardly any idea at all of how to build it.

People had argued about building roads in the region since the Klondike gold rush at the turn of the 20th century, when stampeding miners took boats to the Alaska panhandle town of Skagway, hiked across the mountains to Whitehorse and boarded paddle-wheel steamers for the trip up the Yukon River to the gold fields. Many would-be road builders argued for a coastal path.

In 1942 it was clear that the military road had to be out of reach of carrier-based Japanese bombers. Moreover, it would have to link to the Northwest Staging Route, a series of highly primitive airstrips built between Grand Forks, N.D., and Fairbanks for ferrying Lend-Lease planes to the Soviet Union.

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In the end, planners sketched an inland route and assigned five regiments of the Army Corps of Engineers to move men and equipment over frozen rivers, set up base camps and get ready to build.

But how? Heath Twichell, a historian and retired Army colonel, tells the story in “Northwest Epic” (St. Martin’s Press, 1992), a widely cited history of the road. Their first step, he writes, was to dispatch a surveyor or two on horseback or dogsled, accompanied by a native guide. They would stake out a possible route for the road.

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Behind them came men on bulldozers who knocked down the trees. Behind them came another bulldozer team to clear debris and build bridges and culverts. Next came surveyors to mark out centerlines and edges, then a team to fill low spots and construct drainage ditches.

The men dragged sleeping bags, food and supplies behind them. The entire enterprise was called a train, and the men on it worked seven days a week, in long summer days for 14 hours or more at a stretch.

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They suffered “the most extreme conditions imaginable,” as a private contractor put it a year later while seeking civilians to work on upgrading the new road and on a subsidiary oil pipeline built nearby.

“Temperatures will range from 90 degrees above zero to 70 degrees below zero,” read a help-wanted ad. “Men will have to fight swamps, rivers, ice and cold. Mosquitoes, flies and gnats will not only be annoying but will cause bodily harm.”

At times progress was agonizingly slow, especially when a train encountered muskeg. Eventually, Dr. Twichell writes, the engineers learned to cut trees and plants by hand in muskeg areas, to minimize disturbance to the soil. Small plants were left where they fell; the trees they laid crosswise over the road’s path, to create a surface ribbed like corduroy. Over this surface went loads of soil and gravel. The corduroy stretches were passable — barely.

When they encountered melting permafrost, the builders did much the same.

But when possible, they built around these obstacles, ending up with a road so filled with twists and turns that people assume even today that it was designed to enable truck drivers to evade attack from the air. In a process still under way, engineers have straightened most of these curves, though occasional S-bends remain to tell drivers when they are on the road’s pioneer path.

As a result of this work, the road is miles shorter than it was in 1942. By its official mileposts, Whitehorse is at Mile 918; in fact, it is only 884 miles from Dawson Creek.

Changing Landscape

Then as now, the Yukon was sparsely populated. But while today Whitehorse, the capital, has about 24,000 people, in 1941 the entire territory had only a few thousand, many of them members of indigenous tribes who had never seen or heard of bulldozers until they woke one day to find them chomping through the landscape.

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The results were highly disruptive, and not just because the soldiers, desperate for fresh food and amusement, hunted with Army weapons and fished by setting off explosives in rivers or lakes. While many native people managed to ignore the road project, others worked for wages for the first time in their lives.

“The project disrupted the traditional subsistence cycle by introducing high-paying jobs that lasted a short time but made people dependent on the wage economy,” said Michael Gates, a Yukon author and historian who lives here. “All of a sudden rules were made about what they could or could not do. The Yukon was colonized and the indigenous people were marginalized.”

By the summer of 1943, the Japanese had been driven out of the Aleutians and the tide of war in the Pacific had turned. Shipping routes to Alaska were safe again, and the safety of North America no longer depended on the road.

The oil road, built at enormous effort and expense, found itself under investigation as a waste of time, money and effort.

Today it is maintained “to a very low standard,” Mr. Hidinger said. “It is very twisty, and the drainage is terrible. It’s not reliable enough for any kind of industrial use.” But with mining for lead, gold and other metals on the rise, “it’s starting to become more interesting,” he said.

As for the Alcan itself, after the war ended, Canada paid the United States about half the cost of building the Canadian portion. Control of the highway passed in 1946 to the Royal Canadian Engineers and subsequently from Canada’s central government to British Columbia and the Yukon.

Today it is impossible to imagine life in Whitehorse without it.

Without the road, Mr. Hidinger said, there would be far fewer people here, they would get around largely by boat (and only in warm weather) and they would be a lot less healthy. “They would be living on canned food,” he said. Today “you can put something on a truck in Vancouver and it’s here in 30 hours.”