If you want to know why Boris Johnson has announced a madcap plan to close Heathrow and relocate all its functions to a new airport, just take a look at this map:
The three proposed sites the Mayor has highlighted to take over the functions of Britain's biggest airport are Stansted, already the third biggest airport in the London area; a new airport on the Isle of Grain, in Kent; and a new airport on a new island somewhere in the outer Thames Estuary. Notice the common thread between all the proposals? None of them are in Boris' constituency.
Airports are, as a rule, unpopular in the local area. They are noisy, ugly, noisy, crowded, noisy, obstructive and really, really noisy (source: seven years in the Heathrow flightpath). They bring a lot of jobs to the area, which offsets part of the hatred, but fundamentally they are an example of the sort of tradeoff the state has to make: lives get a lot worse for a small number of people to make things a bit better for a lot of people. Someone has to live next to an airport, and, for the last 50 years, a lot of them have been in south west London.
So it's a very good move, politically, to move an airport from a place filled with people who can vote for you to a place filled with people who can't vote for you. If any of Johnson's proposals go ahead, there will be a lot of angry people from Essex or Kent. But none of those people can vote for Boris – while all of the people in the new aeroplane-free suburbs of London can (and given many of the seats there are Tory/Lib Dem marginals, probably will).
None of the marvellous political calculus involved changes the fact that shutting Heathrow would be a monumentally stupid idea. The entire transport infrastructure of south west London, and much of the transport infrastructure of the South East in general, is geared towards getting 70 million people to and from the airport every year. There are three tube stations, two rail connections, two motorways and a whole load of businesses built based on the idea that there will be an airport in Heathrow. Conversely, the Isle of Grain has one single carriageway and a goods line, and Boris Island doesn't actually exist yet. And that's not even getting into the fact that both the Kentish proposals call for using excess capacity on HS1 which would be put to better use bringing further EU trains through the Channel Tunnel, putting flight and rail connections in direct competition unnecessarily.
We've been calling for more transport infrastructure to be built for years now – but that doesn't mean we ought to junk what we have. If Boris wants to win round south west London to his cause, he's going to have to find a better way than this to do it.