London is a city that rarely entered the twentieth century, and to find this stretch of motorway little more than a stone’s throw from Marble Arch is a poignant reminder of what might have been. Join it by travelling west along Marylebone Road, not far from 221b Baker Street, Sherlock Holmes’s notional address. The WestWay is a continuous overpass some three miles in length, running towards White City, home to BBC television, and then to Shepherd’s Bush, where Pissaro’s house is still standing. By international standards the WestWay is unremarkable, and affords a view of some of the most dismal housing in London. But that is not the point. Rising above the crowded nineteenth-century squares and grim stucco terraces, this massive concrete motion-sculpture is a heroically isolated fragment of the modern city London might once have become. There are few surveillance cameras and you can make your own arrangements with the speed limits. Corbusier remarked that a city built for speed is a city built for success, but the WestWay, like Ankor Wat, is a stone dream that will never awake. As you hurtle along this concrete deck you briefly join the twentieth century and become a citizen of a virtual city-state borne on the rush of radial tyres.