I have the M4 motorway in my Port Talbot back garden

Narelle Cowan and Aimee Lipley’s gardens have a few things in common.

Both have beautiful vegetation, seating for al fresco dining – and one of the UK’s busiest high streets.

On the main road between London and South Wales, around 75,000 vehicles travel over the concrete monolith, which is suspended 45ft (14m) above the pavement every day.

“I call it my free gazebo,” said Amy. “I love it down here, when it rains you can go to the shops without washing your hair.”

The dual carriageway in the sky is one of the original sections of the M4 and a legacy of the 1960s freeway boom in post-war car ownership.