Leading article: A new runway is the last thing that Heathrow needs

After much delay, the Government, it seems, is about to announce that it intends to proceed with a third runway for Heathrow airport. The series of postponements - the latest last weekend - resurrected the charge against Gordon Brown – largely forgotten during the financial crisis – that he was a ditherer. In this case, though, hesitation would have been an improvement on a wrong decision.

Figures released yesterday supplied additional reasons to question further expansion at Heathrow. In 2008, the number of people using London's three biggest airports – Heathrow, Gatwick and Stansted – declined for the first time since the scare precipitated by the attacks of 9/11. Passenger numbers fell at all three, with Gatwick and Stansted each down by more than 13 per cent. The drop at Heathrow, where the initially troubled Terminal 5 is now in operation, was smaller.

Together and separately, the numbers gave the lie to two arguments that supporters of Heathrow's third runway had long taken as given. The first was that air travel to and from Britain wasbound to increase into the future. The second was that London and the South-east were approaching aviation overload, or had already reached it.Even a slight fall, though, produces spare capacity. Declines of more than 10 per cent are substantial. Would it not have made more sense from every point of view to switch more flights to, say, Gatwick – if there really was a runway shortage at Heathrow? It would be cheaper, quicker and infinitely less disruptive than building a new runway from scratch - and would not, if present trends continue, require expansion at Gatwick either.

High speed trains, as advocated by the Conservatives and the Liberal Democrats, should have been another part of the solution. It appears that the Government will temper its support for a third Heathrow runway by including a new high-speed train link to St Pancras. The point of new rail services, however, should be to replace flights, where possible, not to supplement them. Of course, advocates for Heathrow's expansion – chief among them business leaders, but also trade unions protective of local jobs – never accepted that Heathrow was just another London airport. They insisted that, as the destination much preferred by scheduled airline companies and business passengers, Heathrow's competitors were less other London airports than Paris, Amsterdam and Frankfurt. If Heathrow could not remain competitive, they argued, then Britain, and London in particular, would be a serious commercial loser.

How much business Heathrow really brings to London and the region, though, has long been questionable. Because it is seen, notably by British Airways, as a "hub" airport, with many flights connecting from elsewhere, a large proportion of its passengers only transit through Heathrow. Such a "hub" could be placed almost anywhere, including – as the Mayor of London proposes – in the Thames estuary. It does not have to be in a densely populated area, where aircraft disturb many millions of people.

Fierce local opposition, given voice at Westminster by MPs who risk losing their seats over the runway issue, has reinforced protests by "green" campaign groups. And the Government's decision supposedly recognises the need for environmental safeguards. But experience teaches that assurances over carbon emissions and noise are unlikely to be worth the paper they are written on.

If, as expected, a decision is announced today, the planning inquiry will become the next battlefield. With environmental considerations, high-speed rail opportunities and a slowing of aviation growth all pointing so clearly in the same direction, the Government seems about to place itself on the wrong side of transport history.