The man whose business is travel: air travel is changing the way that engineers change the world

Air travel does more than allow you to swap continents in a few hours, transact business and see the world. It is also changing the way that engineers change the world.

The handy thing about aircraft is that they can go pretty much anywhere - unlike roads or rail, they require relatively little terrestrial infrastructure. And the more affordable that flying becomes, the less the incentive for great engineering projects.

Had, in 1986, the fall in short-haul air fares been forseen, the Channel Tunnel would never have been built. At the time, the cheapest flight between Britain and France cost close on £100, and fares to countries such as Italy were prohibitive. When the tunnel opened in 1994, Eurostar's lowest fare from London to Paris was £95. It has sunk to £59, due to the intense competition from the airlines. The investors who paid for the hole in the ground between Folkestone and Calais to be dug are counting their losses, and prospective backers for other great schemes are looking cautiously at the prospects.

Without enough passengers paying premiums to avoid sea crossings, bridges and tunnels cannot provide a sensible return on investment - even between locations separated by tantalisingly short stretches of sea. While the bridge-and-tunnel link between Copenhagen in Denmark and Malmo in Sweden has proved extremely useful for business travellers, and the soon-to-open link from Italy's mainland to Sicily will be a boon for everyone except the ferry operators, other great schemes may never get off (or under) the ground.

Tarifa in southern Spain, close to Gibraltar, is only eight miles from the African mainland. There has long been talk of a tunnel (a bridge would prove impractical over one of the world's busiest sea lanes). A link from Morocco to Europe sounds enticing, but even with the economic accelleration it would trigger the density of traffic cannot justify it. More exciting yet are the rumours of a connection from the southern tip of Yemen to the Horn of Africa; it is thought that Middle East oil money would pay for the connection from the Arabian peninsula to Africa. Geo-politics, rather than funding, causes the obstruction; a connection to strife-torn Somalia does not look a safe bet.

Across the Indian Ocean, who would not want to travel on the maiden voyage of the Delhi-Colombo Express? The stretch of water between the southern tip of India and the island of Sri Lanka is tantalisingly close, with a string of islands to ease the engineering task. Yet besides the inter-communal conflict in Sri Lanka, the spread of low-cost flying in the sub-continent militates against such a plan.

Tasmania could finally join mainland Australia if a massive project connected the island with Victoria, but all the evidence is that Australians are far happier flying - indeed, ferry services have been cut back recently.

Any sources of optimism for those in the civil engineering business? Yes, but you have to look to Latin America. The final 100 miles or so of the Pan-American Highway between Panama and Colombia is profoundly controversial - partly among those concerned about the environmental and cultural impact of completing the world's longest highway, but mainly because of worries about smuggling from the source of much of the world's cocaine. Indeed, the US would probably pay for the Darien Gap to remain.

The one bright hope for terrestrial transport is South America's 2,500km Inter-Oceanic Highway, designed to connect the Atlantic and Pacific Oceans. It is due to open in 2008: the last stage, from Puerto Maldonado to the Pacific coast of Peru is being completed. I cannot wait to make the trip, but I am likely to be a rarity: any of five flights a day will get you from Sao Paulo to Lima in five hours flat.