Diving to attempt to get back into the cab the driver slipped crashing to the ground.
The vehicle was now out of control, with an elderly lady still inside.
At that moment local man Shane Clark was leaving an appointment when he saw what was happening.
“I came out of St Vinnies and I was talking to my friend,” Mr Clark said.
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“The next minute I heard a lady scream and I saw the cab rolling backwards and she was running to catch it.
“I could see an old lady inside the vehicle.
“The driver tried to stop it, but she had fallen over, so I jumped up and just ran for it.
“I actually wasn’t too sure if it was going to hit me or not but I took a chance and I jumped in and stopped it.
“The old lady in the back was shaking, I think she thought she was going to die.
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“So I put it in park and checked everyone was okay.
“It must be the adrenaline, but I was shaking, too.”
Mr Clarke said it all happened quickly, but there was also a point, once he had entered the cab, that he could not find the hand-break.
“It wasn't in the normal place,” Mr Clark said.
“It was one of those vans where it's down by the floor, but luckily I managed to find it.
“The taxi driver thanked me, I think she felt a bit silly, but I don't think it was her fault.
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“Those new-style gear-sticks are so easy to knock.
“I think she might have knocked it out of park when she got out.
“The old lady calmed down a bit, too, once she was out of the taxi. She thanked me, too.
“But they don't need to thank me.
“I had no time to think, I just did what I did.
“And if I found myself in the same situation again I'd do exactly the same again.”