The route is derived in several places from the military roads constructed through the Highlands by General George Wade and Major William Caulfeild in the 18th century, along with later roads constructed by Thomas Telford in the 19th. The modern route is based on that designed by Telford, but with a number of improvements primarily dating from the 1920s and 30s. These include a diversion across Rannoch Moor and another around Loch Leven, which was subsequently replaced by the Ballachulish Bridge.