Flying cars may be 25 years or more away

That is the verdict of engineers from the US space agency and aeronautical firms, who envision future commuters travelling by "skycar".

These could look much like the concept skycar shown in the picture, designed by Boeing research and development.

However, such vehicles could be many years from appearing on the market.

When you try to combine them you get the worst of both worlds: a very heavy, slow, expensive vehicle that's hard to use Mark Moore, Nasa Langley Research Center

Perhaps one of the best publicised efforts at developing this type of craft is the Skycar concept produced by Moller International.

The Davis, California, company has already flight-tested tethered prototype models and hopes to receive airworthiness certificates so that a limited number of its cars can be made available in the next few years.

'Gee-whiz'

These types of vehicles present a number of challenges for designers.

Ideally, the car-plane combinations would not be expensive and would not require the skills of a trained pilot to fly.

"When you try to combine them you get the worst of both worlds: a very heavy, slow, expensive vehicle that's hard to use," said Mark Moore, head of the personal air vehicle (PAV) division of the vehicle systems program at Nasa's Langley Research Center in Hampton, US.

Boeing is also considering how to police the airways - and prevent total pandemonium - if thousands of flying cars enter the skies.

"The neat, gee-whiz part is thinking about what would the vehicle itself look like," said Dick Paul, a vice president with Phantom Works, Boeing's research and development arm.

"But we're trying to think through all the ramifications of what would it take to deploy a fleet of these."

Past proposals to solve this problem have included artificial intelligence systems to prevent collisions between air traffic.

Nasa is working on flying vehicles with the initial goal of transforming small plane travel.

Moller sees its first vehicles having niche uses, such as military applications

Small planes are generally costly, loud, require months of training and lots of money to operate, making flying to work impractical for most people.

But within five years, Nasa researchers hope to develop technology for a small plane that can fly out of regional airports, costs less than $100,000 (�55,725), is as quiet as a motorcycle and as simple to operate as a car.

Although it would not have any road-driving capabilities, it would bring this form of travel within the grasp of a wider section of people. Technology would automate many of the pilot's functions.

This Small Aircraft Transportation System (Sats) would divert pressure away from the "hub-and-spoke" model of air travel.

Hub-and-spoke refers to the typically US model of passengers being processed through large "hub" airports and then on to secondary flights to "spoke" airports near their final destination.