"Unknown road," warned the stranger's satnav. Diverted from the familiar A303 by a sudden closure on a dark evening, I joined another baffled motorist at a tiny junction with an unhelpful signpost indicating an unexpected place name such as Longbridge Deverill, or perhaps it was Marston Magna. It was a shock to be faced so abruptly with the A303's hinterland, hitherto unknown to me and, I suspect, many of the drivers of the vehicles – sometimes more than 30,000 – pounding over what Tom Fort calls "the highway to the sun".
We race through King Arthur's Camelot or, to be precise, between North and South Cadbury. We idle in neutral at the jams which build up round the end of dual carriageways where two lanes squash themselves into one. We accelerate nervously into the third lanes – an invention of the devil – with the centre lane shared for overtaking by traffic racing in either direction.
This trunk road consists of 90-odd miles from the junction with the M3 in Hampshire to Annie's Tea Bar in Devon, where it joins the A30. Unlike the alternative route via the M4 and M5, which the elderly maps in this highly enjoyable book do not deign to feature, it brings us elegant curves and the wonderful sweep of Salisbury Plain.
Fort has explored this A-road and its environs by car, bicycle and foot. The A303 takes the scenic route, with historical detours and geographical byways. Wincanton is one of the few towns to be twinned with a fictional place, Ankh-Morpork from Terry Pratchett's Discworld series.
The A303 officially dates back to 1937, when a series of existing roads was rebranded and given this single number. Joining up the dots did not, of course, improve the driving conditions, but it was not until 1962 that a substantial stretch (near Amesbury) was "dualled", with the first two bypasses opening seven years later. The last major undertaking was the Bourton bypass in 1992. That's it. The proposed Stonehenge tunnel is untunnelled and the A303 remains unfinished business. "We rather like it like that," says Fort. Keep his book in the glove compartment, to read at points where two lanes squeeze into one.