It is ridiculous for people to move near an airport and then complain about the noise. I wish there were more environmentalists on this forum to give that side of the story. But from what little I have heard from them, their position is absurd. The lost economic value from preventing LHR expansion is huge.
I wrote an Environmental politics seminar paper on this so maybe I can shed some light on this subject. As I saw it, the whole thing is, more or less, a double-edged sword. If LHR does not, for example, build T5 they stand to lose a significant amount of revenue to Continental airports which are far less hemmed in. BAA/BA have promised that they will build T5 and not a thrid runway.
For their part, enviromentalists (especially the West London Friends of The Earth) don't buy into BAA/BAs line. They say that the history of LHR is one of broken promises. Therefore, they are convinced that construction will not end with T5. Rather, they are convinced that a 3rd runway will follow in short order succeeded by a sixth terminal. Furthermore, they argue that because West London is now one of the most congested areas of London that building T5 will only excacerbate the situation. Moreover, they argue that the Perry Oaks Site (the site on which the proposed terminal will be built) which acts as a wildlife refuge, the Twin Rivers, and the Colne River Valley are all far too important to Britain to be destroyed in order to bring about T5 construction as well as the widening of the M4. Finally, they argue that, because the airport has already surpassed the number of aircraft movements that BAA claimed LHR would be at in 2008 with the opening of T5, this is a clear indication of the fact that BAA and BA have no interest in curbing the number of flights at the airport. Thus, they argue that flight movements will continue to increase affecting local noise pollution, air pollution, and water pollution.
This is a very, very abbreviated version of the problem. My paper was roughly 27 pages long and IIRC the governments report on the matter was some 1000 pages long. There's no clear answer to the problem. The only clear thing in this whole mess is that both sides have compelling arguments to back up their position.
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