Look at a relief map of the central Highlands and Drumochter is the obvious flaw - a U-shaped notch where the Grampian Watershed sags low enough for engineers to bother with it. Between Glen Coe to the west and the Cairnwell Pass to the east, a span of nearly 100 kilometres, no other gap proved suitable for a major road. Successive ice ages did the rough cutting, gouging out the broad floor where the headwaters of the Spey and Tay reach deepest into the Gaick Plateau. The River Truim drains north toward Inverness; the River Garry flows south toward Perth. Loch Ericht, just to the west, marks where the glaciers cut harder still, but its trench is too deep and too steep to make a road of. Drumochter, by comparison, is the polite option.

The route is pre-Roman, walked by travellers and trading parties long before maps existed. General Wade's troops paved a military road through it between 1728 and 1730, part of the Hanoverian project to bind the Highlands by stone and discipline after the failed Jacobite rising of 1715. Cattle drovers brought their herds south through the pass to the Lowland markets at Falkirk and Crieff, sleeping rough on the hillsides. The Highland Main Line railway, completed in the late nineteenth century, climbs to a summit of 452 metres just south of Dalwhinnie - the highest mainline railway summit in the United Kingdom. For decades the RAF used the pass as a low-level flying training route, fast jets skimming the heather between Beinn Udlamain and A' Bhuidheanach Bheag.

Winter at Drumochter is a different country from winter ten miles north or south. From November through March the road operates under standing patrols, and snow gates near Dalwhinnie and Dalnacardoch can swing shut without warning when blizzards blow in off the Monadhliath. The nearest settlement of any consequence is Dalwhinnie itself, ten kilometres to the north - a single street, a railway halt, and a famous distillery. Otherwise the pass is empty in the human sense: just grouse moors, deer fences, mountain hares, and the long lines of pylons that march beside the road. The summit signpost marks the boundary between Perth and Kinross and the Highland Council area, and it reads, in two languages, 'Welcome to the Highlands / Failte don Ghaidhealtachd' - though the geological Highlands extend much further south, all the way to the Highland Boundary Fault at Dunkeld.

The folk song 'Tha Sneachd air Druim Uachdair' is older than the modern road, older than the railway, older than the maps. It belongs to a time when crossing the pass in winter meant gambling with your life. The melody is plain, almost flat, the way songs sometimes are when they exist mostly as practical information set to a tune you can hum while you walk. Singers of the song still preserve a memory of what this place meant before tarmac and rotary snowploughs - the gap you had to cross, the weather that decided whether you would. The same wind still funnels through the pass in February. The snow still arrives in the same drifts. The road is wider and the trains run on schedule, but the pass itself has not changed its nature.