The Orbital Moatway was a structure that surrounded London, made of decayed tarmac (so likely, a former road.). It marked the

The 'Moatway' today
boundries of London's territory, and was even fortified during the Scriven rule of London. It was still used after the Scriven Uprising, by the Trained Bands (the part-time militia that protected London pre-Movement rule).
It used to be the Orbital Motorway for London, pre-Sixty Minute War. It was often clogged with cars (a type of vehicle mistaken for a 'mobile neighbourhood') then - instead of fortifications.