The 59 service - operated by Stagecoach - was following its usual route around Royston on Monday afternoon when the driver apparently missed his turn onto Cross Lane and carried on towards the bridge on Lund Hill Lane.
The bus then smashed into the bridge and its entire roof was sliced off. Similar incidents have occurred several times over the years.
Luckily, there were only two passengers on the top deck and neither’s injuries were fatal - but 52-year-old John Cook, of Shaw Street, near the town centre, needed hospital treatment and believes he’s lucky to be alive.
He told the Chronicle: “It seemed like it was a usual bus journey because we were going on the normal route - I was on the back seat just slouched down a little bit.
“Then all of a sudden, I felt a massive jolt and I could see the bus was going underneath the bridge.
“I looked up and the roof was coming towards me and I could hear the windows popping - luckily because I was at the back of the bus, the roof went just over my head.
“The bus then suddenly stopped and I ended up being thrown forward before I smashed my head.
“There was a massive cut - I didn’t even know what had happened at first.”
When John looked around on the top deck, he saw the headrests on many of the seats had been completely ripped off by the roof, making him realise how fortunate he was.
“Luckily the woman at the front of the bus managed to see it coming so she dropped down onto the floor and avoided being hit,” he added.
“The driver came running upstairs in a panic and he was really apologetic about what happened.
“I wanted to phone the emergency services but when I got my phone out it was just covered in blood because of how much was pouring out of my head.
“When I was looking around, I saw the damage that was caused and that’s when I realised that this could have been a lot, lot worse - there was blood everywhere.
“The driver then rang the emergency services. He came back upstairs with towels to stop the bleeding before I was taken downstairs.”
John has praised the work of the emergency services who told him they couldn’t believe he escaped with the sort of injuries that he had.
“When they arrived, there was still a fountain of blood pouring from my head,” said John.
“The fire service and the ambulance staff said they don’t know how I managed to get away with it all - I’m lucky to be alive.
“They were amazing and I ended up staying in Barnsley Hospital overnight before being discharged on Tuesday afternoon.
“I really think that the adrenaline kept me going because I could have died.”
South Yorkshire Fire and Rescue confirmed they were called at 2.59pm on Monday and bosses told the Chronicle that John had ‘quite a bad head injury’ and was given urgent medical attention.
A spokesperson added: “When they arrived, they found a double-decker had gone under a low bridge - ripping off the entire roof.
“They searched the top deck and confirmed nobody was up there still, before tending to two passengers that were injured - one woman and one man.
“Crews gave him first aid until the arrival of the paramedics and then helped get him from the bus to the ambulance.
“We left the scene at around 5pm after stabilising the vehicle for recovery by the bus company.”
A spokesperson for Stagecoach told the Chronicle: “We can confirm that one of our services was involved in an incident on Lund Hill Lane at Royston on Monday.
“There were four passengers on the bus, two of whom have been injured.
“Emergency services attended the scene.
“Our first thoughts are with those who have been injured.
“Safety is our absolute priority and we are fully supporting the investigation into the circumstances of the incident.”