About 30 kilometers east of El Calafate, northbound RN 40 covers 596 kilometers of rugged gravel road—scheduled to be completely paved within four years or so—in the Andes’ arid eastern foothills, before arriving at the cow town of Perito Moreno near Lago Buenos Aires, the oasis of Los Antiguos, and the Chilean border town of Chile Chico.

From the Bolivian border near La Quiaca to its terminus near Río Gallegos, RN 40 is Argentina’s great unfinished interior highway. Some segments of “La Cuarenta” in the central Cuyo provinces are smoothly paved, while others in the Andean northwest are rough and rugged. None of those, though, enjoys the notoriety of the segment between the El Calafate junction and the town of Perito Moreno, on the cusp between the Patagonian steppe and the icy southern Andes.

For Argentines and foreigners alike, La Cuarenta has become the standard for adventurous driving and cycling thanks to its secluded Andean lakes, isolated estancias, plentiful wildlife, and rare sights like the pre-Columbian rock art of Cueva de las Manos.

When I first drove the 594 kilometers in early 1991, I saw only three other vehicles in four days, and services were almost nil. Since then, traffic has not exactly burgeoned, but the summer season sees a small but steady procession of motorists, motorcyclists, and bicyclists.

It’s clearly not for everyone, though, and a trip up or down La Cuarenta requires planning. With accommodations and supplies few and far between, bicyclists and motorcyclists must carry tents and cold-weather gear, even in midsummer, and plenty of food. Detailed maps, like ACA’s newest regional sheets, are essential. Preferably, automobiles should carry at least two spare tires.

Also carry extra fuel—between El Calafate and Perito Moreno, the only dependable supplies are at El Chaltén (a 90-kilometer detour), Tres Lagos, Gobernador Gregores (a 70-kilometer detour, but it sometimes runs out), and Bajo Caracoles (which also sometimes runs out). Some tourist estancias will sell gasoline to their clients or in an emergency, but don’t count on it.

Road hazards are numerous. Bicyclists and motorcyclists must contend with powerful Patagonian winds that can knock them down in an instant, and deep gravel adds to the danger. Even high-clearance vehicles are vulnerable to flipping on loose gravel, especially when braking suddenly, and 50-knot gusts make things worse. Though four-wheel drive is not essential, some drivers prefer it to avoid fishtailing on gravel.

Chipped, cracked, and even shattered windshields are par for the course on RN 40 and other graveled roads. Normally, rental-car insurance policies do not cover such damage, and replacements are expensive in Argentina. Approaching vehicles usually brake to minimize the possibility of such damage, but some drivers find they need to play chicken to slow down an onrushing pickup truck or SUV.

The big news is that within a few years this segment of RN 40 will be paved and may be rerouted to pass through Gobernador Gregores.

If driving or cycling doesn’t appeal to you, but you still want to see the loneliest highway, summer bus and minivan services now connect El Calafate and El Chaltén, at the south end, with Perito Moreno and Los Antiguos at the north end of the province, and with Bariloche.

Still, this segment of “La Cuarenta”— running from Río Gallegos almost to the Bolivian border, RN 40 is Argentina’s longest interior highway—has a mystique all its own. Some love it, some loathe it, and others are ambivalent, but no one forgets it.