Heathrow Airport is the world's fourth busiest airport, the busiest airport for international flights and by far the busiest in Europe. It was built at the latter end of World War 2 to be a civilian airport and grew quickly to be London's major airport. Despite efforts such as Gatwick Airport and various plans to build an airport in the Thames Estuary, Heathrow has been expanded several times, most notably with Terminal 5, which required the widening of the M25 between the M3 and M4, as well as a motorway spur to the new terminal. Plan exist for a third runway, which would put the M25 in a tunnel, add a collector-distributor set-up between junction 15 and a radically reworked junction 14.
The Airport proper is surrounded by Perimeter Roads (Northern, Western, Southern, Eastern) and other roads mostly begin with directional letters or 'C' for Central.
Heathrow has several major road tunnels, though only the Heathrow Tunnel is open to the public. The Cargo Tunnel used to be, though now isn't. The Heathrow Airside Road Tunnel requires going through passport control and security to use.