The bin lorry is believed to have struck the first pedestrian outside Glasgow's Gallery of Modern Art, before careering down Queen Street and through traffic light road junctions before coming to a stop after hitting the Millennium Hotel in George Square.

Investigations into the incident are still in the very early stages, but one element experts will examine is how the vehicle travelled around 150 yards in an almost straight line before it came to rest.

Eyewitnesses reported seeing the driver of the Glasgow City Council refuse vehicle slumped over at the wheel, prompting speculation that he may have suffered a heart attack or other physical seizure. He is now being treated in hospital.

A spokesman for the Institute of Traffic Accident Investigators said that vehicles, even large ones, can carry on for a considerable distance if there is an "unintended acceleration", which could be caused by a driver falling ill - and perhaps going into a spasm - with their foot on the accelerator, or by someone inadvertently pressing the accelerator rather than the brake.

The man, who asked not to be named, said: "Without speculating on this incident, there might be a number of causes for a vehicle to travel a considerable distance.

"If there is no external influence on the steering direction of the vehicle, then it will continue in a straight line.

"An external influence might be a driver applying steering, or the wheels coming into contact with something like a kerb.

"Regarding the distance travelled, one would expect a vehicle to slow if the driver's foot had been removed from the accelerator, due to engine braking. And of course if the brake were applied one would expect it to slow."

The spokesman said that in the eventuality of a brake failure a driver can also slow a vehicle by changing down through the gears.

He added: "But there are air brakes on those (bin lorries) and vehicles are fitted with a fail-safe mechanism. In air-brake system failures, brakes will come on automatically.

"Had there been a failure in the air brake system the air brakes should automatically apply, but if it has carried on all this distance that would not appear to have been a problem in this case.

"So gravity and engine braking would slow it down over a relatively short distance, if acceleration was not applied. And obviously brakes being applied should slow it down over a relatively short distance."