EXPERTS say driver error may not be the reason for a runaway bus causing chaos on a city centre street.

The number 60 First bus careered 100m down Renfield Street on Monday around 3.45pm, after the driver reportedly got off to buy a drink.

First Bus bosses are now carrying out safety checks on the vehicle as part of an investigation into the incident.

A shopkeeper in RF News said the driver had been in to a bakery, and his shop, before noticing the bus was rolling down hill without him.

Read more: Driver suspended after runaway bus causes mayhem in the city centre

Transport sources speaking to the Evening Times suggested if a handbrake had not been applied at all the vehicle would have started moving immediately.

One source, from a firm which trains bus drivers for work all across Scotland, said: “I don’t know how long a brake would take to come off entirely but I’d think it would have rolled practically the moment you left the bus if it was on a slope and wasn’t on at all.

“It’s a possibility that something has gone wrong with the handbrake.

“It is possible that he has done everything correctly.

“We train drivers to switch the engine off, put the handbrake on and lock the doors before leaving the vehicle.”

Another transport source added: “I don’t know if getting off the bus is part of the policy of First Bus but I would have thought it would have started to move much sooner if he had been in two shops.

“It may not have been driver error.”

Read more: Driver suspended after runaway bus causes mayhem in the city centre

Police and First Glasgow are working on an investigation into the incident which saw two people taken to hospital.

Three buses and a blue Renault Twingo were ploughed into by the rogue vehicle while onlookers, who were enjoying the bank holiday, looked on in shock.

Passersby described hearing a loud bang before noticing the bus crashing in to vehicles ahead.

The runaway bus eventually came to a halt close to St Vincent Street after crossing West George Street and hitting three buses.

Passengers on the Stagecoach bus which bore the brunt of the crash were screaming as they tried to get out of the emergency exit.

Read more: Driver suspended after runaway bus causes mayhem in the city centre

A First Glasgow spokesman said: “Our investigation is already underway, which includes reviewing the vehicle’s engineering and safety records.

“We’re working closely with Police Scotland, however, it will take some time to establish all the facts.

“It is therefore too early at this stage to speculate on the cause of the incident.”

Police Scotland said the investigation is ongoing.