About 220 pregnant sheep blocked an A-road after a country gate was left open, allowing them to escape.
Edward Albutt is part of a family of generational farmers who run the Tewkesbury Nature Reserve, Gloucestershire, where his herd of sheep help graze the land.
At about 03:00 BST on Sunday, he was awoken to calls reporting half his flock were wandering through Walton Cardiff, while the other half was blocking the A38 bypass.
The sheep were returned safely after an hour of herding, but Mr Albutt is urging people to be more diligent about shutting gates behind them.
Charlie AlbuttUpon learning of the escape, Mr Albutt said he immediately felt “anxious” over their fate.
“They walked under the bypass, up the slip road and onto the Tewkesbury bypass,” he said.
“Thankfully no traffic was coming at the time, because it could have been catastrophic.
“You don’t expect to see a flock of sheep in the middle of a busy road, it could have ended quite differently.
“A car could have ploughed into any one of them, it doesn’t bear thinking about.”
Georgina MinchewMr Albutt says this is not the first time this has happened, and the escape attempts are unfortunately becoming “more common”.
“If the sheep come along and find the gates even slightly open, they’ll make their way out for the fun of it.
“I was relieved that none of them had been killed, but that quickly turned into frustration and anger that someone has not bothered to close the gates again.
“Please, please, when you walk through a gate, just look behind you and check that it’s latched,” he added.