As the A1 climbs out of London, it is crossed at a high level by Hornsey Lane. Originally this section of road was planned to be in a tunnel rather than a deep cutting. The tunnel was propsed in 1809 which would have made it the earliest road tunnel in Britain. Although construction started, the tunnel collapsed in 1812 and was converted to the cutting we see today.
The tunnel lives on, if only in the name Archway Road - the word Archway being synonymous with tunnel at the time of construction.