Should expansion of London’s airport capacity should take place at Heathrow or Gatwick?  In the last Parliament, the Coalition evaded what an inevitably divisive issue by appointing an Airports Commission and giving its Chair, Sir Howard Davies, firm instructions not to report until after the May 2015 election.  The election took place, and in the summer the Commission duly reported, with a clear recommendation to build a new runway and terminal at Heathrow.

The Commission relied heavily on an elaborate modelling exercise which calculated costs and benefits for the next fifty years.  Little weight should be attached to these calculations.  They project the present into the future with essentially linear trends, even predicting which routes will be flown from where. The modellers world in 2050 is much like it is now, only busier. 

It is only necessary to look at the last century of air travel to see the absurdity of such forecasting.   From Orville Wright’s first flight to the introduction of the jumbo jet  took barely 60 years .  The last half century has been less dramatic – the Boeing 747 is still in service even though its modern equivalents are far quieter and more fuel efficient.  But while the Roskill Commission of the 1960s which reviewed London airport policy got traffic growth projections broadly right, the Commission did not imagine that the growth would come from low cost airlines offering point to point services; still less at fares that in cash terms are barely higher than in 1970.  Even at the last air policy review in 2003,  officials failed to appreciate how the centre of the world economy was shifting east and that Dubai would now be the world’s largest international airport.

But we already know that future changes will be more radical still.  Commercial drones and driverless cars are on the way, and battery technology, static for so long, is now advancing at a pace which will revolutionise not only transport but much else.   These are likely to be the precursors of innovations we cannot yet imagine.  

        In the face of such radical uncertainty, the sensible course is to focus on known facts and give yourself as many future options as possible, not to make up numbers to fill the cells on your spreadsheet.  The advantages of Heathrow are that it is more conveniently located for the majority of its users, and that a single large airport is better for attracting transfer traffic (and hence more users and more connections).  The first advantage is real and enduring:  the second more dubious.  London airport capacity is a scarce resource, and the most cost effective means of expanding it is to persuade a traveller from Chicago to Moscow to transit through Amsterdam or Frankfurt instead.

The plans for Heathrow require the  relocation of a waste disposal facility, putting a section of the M25 into a tunnel, and the immediate opening of a large new terminal. These plans are probably over engineered anyway, in a manner familiar from other large British infrastructure projects.  So new capacity which would cost around £8bn to provide at Gatwick will cost £18bn at Heathrow, making LHR by far the most expensive airport in the world for its users. The new transport links to service a new Heathrow terminal are also much more costly than those for Gatwick. Gatwick’s principal disadvantage – its less central location, twenty-five miles south of London – means that many fewer people suffer disturbance from airport expansion.

That last should perhaps be the decisive factor.  As your plane circles Heathrow one more time, and you curse the decades of procrastination that have characterised the airports policy of successive governments, bear one thought in mind:  If the government decides in favour of Gatwick, capacity expansion might finally happen.