But even before a prototype can be built, the project could be derailed by two 'technical problems': at present there is no way to turn the bus; and a way has yet to be found to stop the vehicle in less than about 11/2 times the length of a soccer field.

In other words, 'Look ma, no steering ... and no brakes'.

Still, city authorities desperate for a cure for traffic congestion are eager to try out the idea. They hope it will spell the end of the noisy, bulky and belligerent buses that clog the capital's roads.

The number of cars in Beijing rose from just over 2,000 in 1949 to one million by 1997.

By the end of last year, a further three million had taken to the roads, and a million more are expected to join them this year.

In earlier, unsuccessful efforts to solve the city's traffic jams, the authorities forced private vehicles to stay home at least one day each week, doubled inner-city parking charges and spent hundreds of billions of yuan on underground railways.

Average rush-hour speeds have slowed to about 24km/h - and that includes vehicles speeding along in the countryside. An average for inner-city traffic is not available.

Zhang Wenbo , science and technology director of the capital's Mentougou district, said the municipal government was giving keen consideration to the 3-D express as a way to revolutionise the city's surface transport. He said a six-kilometre test line was likely to be built in his district.

'We have included [the test line] in our district development plan,' Zhang said. 'If it works, two commercial lines, one of 120 kilometres and the other 60 kilometres, will be built. As far as I know, other districts, especially those in the congested city centre, are geared up to follow.

'Government is not a problem. Money is not a problem. The only problem is that there is no prototype yet. As soon as we see a prototype, we will give construction a go.'

The man who holds the patent for the 3-D express, inventor Song Youzhou , said a prototype would soon be built by China South Locomotive and Rolling Stock Corporation (CSR), one of the country's two train makers.

Song, the chairman of Shenzhen-based Huashi Future Car-Parking Equipment, said CSR's two plants, in Shijiazhuang , Hebei , and Changzhou , Jiangsu , and their host cities were fighting fiercely for the project because they expected demand to be huge.

'We haven't cast the dice yet, but we will do so soon, because the Beijing government is very eager about it,' he said.

Song has more than 120 domestic and international patents to his name, but the 3-D express is his most ambitious invention.

'The idea simply popped into my head when I was stopped dead in a traffic jam more than a year ago,' he said.

One problem is cost. Song estimated that building one kilometre of the tram line over an existing road would cost about 50 million yuan (HK$57.32 million). Much of that would be needed for the reconstruction of existing urban features such as bridges and electricity lines.

One tram with four passenger cabins would cost 28 million yuan. Song said if the mainland's big cities opted for only a modest system, they would still have to find the money for 10,000 of the trams within a decade.

'That's a lot of money,' he says. 'But a kilometre of subway costs 300 million yuan, and subway trains are expensive as well. In comparison, this idea is cheap.'

Another problem is safety. To get passengers safely to the ground during an emergency, Song has also patented a system in which an entire wall of the cabin falls out, forming a gangway that passengers can use to slide quickly off the bus.

Professor Zhang Jianwu , who led a research team at Shanghai Jiaotong University that did a feasibility study of the project, says there are other technical problems.

One is turning. Unlike any of the existing vehicles that run on tracks, the wheels of the 3-D express would not be connected from one side to the other.

That means they would need a completely new turning mechanism - not dissimilar to the digitally assisted all-wheel-drive system on full-size SUVs.

'Of course, no one in the world has tried it before,' the professor said.

Braking is another concern. A four-unit tram, fully loaded with 1,000 passengers, would weigh 140 tonnes and take more than 140 metres to stop - an unacceptably long distance in any city.

Nevertheless, the team gave the project a positive feasibility rating and will send technical data to CSR this month so that construction of a prototype can begin.

'The turning problem can be solved by computer assistance, and a combo-brake could reduce the stopping distance to about 60 metres or less,' the professor said.

'An innovative project must have some flaws, especially when no one else in the world has ever tried it before. There is no other way to deal with them other than to just do it.'

Huashi marketing manager Li Qicheng said he had been contacted by officials from the United States, Canada, Brazil, Argentina, Mexico and India who had expressed genuine interest to the project. He said the overseas market would be four times bigger than the domestic one.

'When I showed Argentinian President Cristina Fernandez de Kirchner our demonstration video in Beijing last month, she was quite excited,' Li said.

'Argentina had just reached a multibillion-US-dollar deal with Chinese leaders to upgrade the country's transportation infrastructure. If some of the money could be spent on the 3-D express, it would be great.'

Despite all the hype from government officials, scientists and entrepreneurs, Beijing residents, especially those who spend a considerable part of their lives behind the wheel, have some reservations about the plan.

Wang Cunyi , a Chaoyang district resident who has been driving for more than a decade, said he would welcome any way to solve Beijing's traffic jam problem once and for all, but only if it was proved to be safe and reliable.

'I will not be the first to go under that thing. I dare not,' he said.

'It looks cool, like in a movie, but I am not Jackie Chan.'

Aiming high

Beijing hopes the ?3D Express? bus (below) will ease rush-hour congestion

A six-kilometre test line will be built in the capital's Mentougou district but a prototype is yet to be built

Cost: 28 million yuan (for a four-compartment bus)

Weight: 140 tonnes (loaded)

Capacity: 1,000 passengers

Maximum speed: 40km/h

Maximum height beneath: 2.1 metres

Bus station: Every 2 km

Width: Two lanes of road (about 15 metres)

Average speed in rush hour: 24km/h

Fuel: Electricity

Number of passengers carried on buses and trolley buses each day: About 14.4 million

Problems: Will need 140 metres to stop Yet to work out how to turn corners

Source: Huashi Future Car-Parking Equipment Company