A fourth lane rejoins at the other side of the M3 junction, and we're about to pass through one of the most congested sections of motorway in the UK. There are only four lanes here - not nearly enough for the volume of traffic passing through - but the real problem is that the next junction has only three lanes through it, a bottleneck that causes queues for much of the day.

This short section from J12 to 11 was opened in October 1980.