The M11 is a motorway built in the seventies to whiz traffic from London to the newly-developed Stansted airport. An additional two-lane section was built north of there, completed in the early eighties, to get traffic to Cambridge and then onto what is now the A14 and then onto the A1.
Short-sighted planning gave the Stansted junction, where the M11 loses a lane, a normal roundabout interchange, at which all the airport traffic got tangled in the traffic lights along with locals trying to get to Bishop's Stortford. Free-flowing links into the airport's road system were built at great expense in 2002, and things are now flowing smoothly at long last.
Three junctions are missing from the start of the motorway - at the London end it begins at junction 4. The unbuilt part would have been a route further into London to reach Ringway 1, a proposed inner ring motorway for London. The link was eventually built to a different design in the 1990s as the A12 between the A406 and Hackney Wick.
In fact there's a lot that never happened with the M11: its route in to London down the Roding Valley was actually the second choice for the motorway and initially the preferred route would have taken it down the Lea Valley instead, passing west of the new town of Harlow instead of east. The change of plan came too late for the planning of Harlow itself, which now has all its industrial areas on the north and west sides of the town, far away from the motorway connection. One of the town's designers described the change as being like building a seaside town and then "moving the sea".