Before the highway, the north got by on short stretches of dirt linking farms and towns to harbours, where steamers called for goods and passengers. It was a poor way to move freight - and a dangerous way to defend a coastline. When Japanese aircraft began attacking northern Australia during the Second World War, the gap became strategic. From 1944, two highways were pushed north from Perth toward Darwin: the coastal Highway 1, and this inland route. Yet sealing the whole length took decades. The final section was not paved until 16 December 1989, an event that made national news. The all-weather road, passable in any season, was the entire point - and it had been the best part of half a century in the making.

Trade still rules the highway. Trucks pound its length at every hour, and the largest of them - road trains, a prime mover hauling multiple trailers - are a force of nature on a two-lane road. Drivers learn to read the whoosh: the pressure wave that hits as one passes or overtakes, demanding two firm hands on the wheel. The road crosses pastoral leases, the wheatbelt, and the great iron-mining districts around Newman, where much of the accommodation is block-booked for mine workers cycling through two-week swings. Newman itself sits where the highway crosses the Tropic of Capricorn, the symbolic line into the tropics, though out here the air feels no different - just hot, and getting hotter as the road climbs north.

This is a road that punishes carelessness. The full route takes about a week with only token stops; tour buses do it in three days by running through the night with relays of fresh drivers, because driver drowsiness is among the deadliest hazards out here. Roadhouses double as petrol stations and rooms, and the rule is simple: never pass one with less than half a tank. Wise travellers carry spare water and spare fuel. Most of all, they avoid driving at dusk and after dark, when cattle and kangaroos drift onto the warm bitumen and their dark eyes throw back no glint of headlight. A collision with a beast at speed can end a journey - or worse - in a heartbeat.

Few roads pass through so many landscapes. South of the iron country, the highway runs through old gold towns and wheat farms. North of Port Hedland - the rust-streaked port where the iron is shipped, and the halfway mark in spirit if not in distance - it bends east and joins Highway 1, threading toward Broome's pearling coast and on into the Kimberley. There the bitumen narrows to single-lane bridges across rivers that swell and flood in the wet, when side roads turn to mud and caravan parks shut for the season. Turn-offs lead to wonders: the red gorges of Karijini, the beehive domes of Purnululu, the Bungle-Bungles. The highway itself stays humble - a thin grey line holding a vast country together.