The road approaching the pass vies with Rosedale Chimney Bank in Yorkshire for the title of steepest road in Britain; both achieve a gradient of 1 in 3 as does the Wrynose Pass which leads on from the Hardknott.
Past the western end of the pass, the road descends through Eskdale towards the coastal towns of Cumberland. At the eastern end, the Wrynose Pass carried the road on eastwards along the county borders to Langdale, whle another road descends through Dunnerdale.
The Romans originally built a road over the pass in the 2nd century to link the coastal fort at Ravenglass with their garrisons at Ambleside and Kendal. The Romans called this road the Tenth Highway and it reaches a height of 1,289 feet at the top of the Hardknott Pass before descending steeply to Dunnerdale and then ascending and descending the Wrynose Pass on its way to Ambleside. The road fell into disrepair after the Romans left Britain in the early part of the 5th century, although it remained as an unpaved packhorse route throughout the centuries.
The War Office used the area for tank training during the Second World War and this completely destroyed the ancient track. After the war a decision was made to repair the damage and rebuild the road with a tarmac surface to give a direct motor route between Ambleside and Eskdale for the first time. However, the Roman route and the modern road do not generally coincide, the Roman route lying generally to the north of the modern road west of the summit, and to the south on the other side.
The pass itself has a series of hairpin bends that can be unnerving for drivers of cars and minibuses (heavier vehicles are advised not to use the pass), especially as the tarmac has become quite smooth in places. Drivers are expected to give way to oncoming traffic that is ascending the pass, as advised by the Highway Code. The pass can be closed for long periods in the winter months as ice makes the bends treacherous.
As the pass leaves Eskdale it passes Hardknott Roman Fort at a height of around 660 feet, and there are a few parking places for drivers who want to stop and look at the ancient site. At the top of the pass the road goes between the fells of Hard Knott and Harter Fell; once again there are few parking places, which are usually taken by fell walkers who wish to start their walk at height or by tourists who want to admire the fine panorama, which on a clear day includes a view of the Isle of Man in the Irish Sea.
The pass featured in the 2007 BBC series Mountain, in which presenter Griff Rhys Jones ascended the Eskdale side, riding pillion on a motorcycle.