Building Heathrow Airport's third runway over the M25 would be "normal practice", an engineering expert has said.
Chris Chalk, who has helped design airports around the world for engineering consultancy firm Mott MacDonald, claimed the runway's slope would be difficult to spot with the naked eye.
Transport Secretary Chris Grayling revealed that the west London hub's new runway could be built over one of Europe's most congested sections of motorway to save money and time as opposed to building a tunnel for the 12 lanes of traffic.
Mr Chalk, who is a member of the Institution of Civil Engineers' transport expert panel, said that international aviation regulations require runways to have a gradient of no more than 1%, and the Heathrow scheme would be "well within that".
He told the Press Association: "People have got this worry that it's going to look like a ski jump or the front of an aircraft carrier. I can assure you it's a long way from that. It would be very difficult to notice it."
He added: "I would say it would be normal practice. There are international standards."
Manchester Airport, Fort Lauderdale in Florida and Charles de Gaulle in Paris already have runways that pass over roads.
Motoring groups stressed the importance of minimising the disruption to drivers on the M25 during the construction of the third runway and once it is operational.
Steve Gooding, director of the RAC Foundation, said: "Everyone recognises that you meddle with one of the busiest and most congested stretches of motorway in Europe at your peril.
"Constructing the runway over the motorway might seem outlandish but shouldn't be ruled out.
"Momentous decisions call for ambitious and imaginative engineering solutions which we need not just for the runway but to tackle existing congestion on the M25 which is forecast to get worse."
An AA spokesman said: "Drivers who regularly have to put up with traffic jams on the west side of the M25 - and are likely to face more traffic congestion with Heathrow expansion - will want to know whether the Government will seize the opportunity to increase road capacity along that section.
"If the original concept envisaged the need for a tunnel to make the airport project viable, you have to question what the consequences will be for Europe's busiest stretch of motorway without one."