Rod Nordlund, Tmes correspondent in and around Afghanistan:

This is the only passable route for heavy truck traffic bringing NATO supplies in from the Central Asian republics to the north, as they now must come.

There are other roads, but they are often single-lane dirt tracks through even higher mountain passes, or they are frequently subject to ambushes by insurgents and bandits. So a tunnel built to handle 1,000 vehicles a day, and until the Pakistani boycott against NATO in November handling 2,000, now tries — and often fails — to let 10,000 vehicles through, alternating northbound and southbound truck traffic every other day.

See also BBC News – US cuts Pakistan aid over jailing of ‘Bin Laden doctor’