Cars mean freedom. Shut the door, turn the key, press the pedal, and off you go, or so it always seems until the next traffic jam. But as every stoplight or motorway tailback reminds us, cars have their limitations. Who has not, at a moment of frustration, nurtured the James Bond fantasy of pressing a button and soaring up and away?

In 1893, HG Wells prophesied a future when, every evening, "a vast cloud of winged figures will begin to erupt from among the grimy roofs of the business houses, and hang for a moment eddying and circling in the air after the cramping labours of the day". In 1917, Glenn Curtiss designed the Autoplane. It had three wings, and the motor drove a four-bladed propeller. It didn't quite fly, but did manage a few short hops.

This was followed by the 1937 Arrowbile, a Studebaker hybrid with detachable wings, also propeller-driven, and Robert Fulton's 1946 Airphibian, a plane that became a car rather than a car that could fly. In 1970, Ford seriously considered marketing its descendant, the Aerocar, designed to fly, drive, then fly again. This even earned its FAA certificate as an aircraft. But that decade's oil crisis put paid to it.

Now flying cars are back. Ninety hopefuls have put down $10,000 each in the hope of being among the first to own a Moller Skycar, with delivery in 2006. Paul Moller, its California-based designer, plans a production model of his M400, a vertical take-off and landing machine he says will make flying to the office routine. The Skycar also has an electric engine capable of 50 mph on land.

Unsurprisingly, flying cars have always been a favourite dream of planners; what better way of clearing city streets and making them safe for pedestrians? Le Corbusier featured them in his City of Tomorrow. Norman Bel Geddes prophesied (in 1937) that "in 1960 ... the average person will be flying about in a small mosquito plane, the roadster of the air". And they featured in Frank Lloyd Wright's Broad Acre City of the same period. But would they really be as delightful as these schemes make them sound?

They will not be cheap. Mr Moller expects his first M400s to cost $500,000. But some think this over-optimistic. Flying cars are not just automobiles, but aircraft as well; and flying a plane is much more complicated and expensive than driving a car. There's the problem of traffic control, for a start. Since there are no traffic-lights in the sky, aircraft need elaborate instrumentation and control systems to help them avoid each other.

Radios that include global positioning systems cost $18,000; aircraft need two. And air traffic control is already overstretched. Realistically, flying cars will have to wait until the autonomous Small Aircraft Transportation System is developed in 2020.

There are other problems. Despite all precautions, accidents happen; the prospect of speeding skycars colliding or suffering engine failure above a crowded street or building is truly terrifying. Then there's the noise. Aero-engines are loud. Imagine lying in bed while your neighbours manoeuvre their skycar out of the garage for takeoff. And what about fuel? Aircraft need a light, energy-rich power source: kerosene. But oil production is about to peak, so the future for optional air travel, though rarely discussed, must be problematic.

So dream on. Flying cars exist. But it doesn't look as if we shall make it aloft for a while.

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