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

Both have pretty plants, chairs for al fresco dining – and one of the UK’s busiest motorways.

About 75,000 vehicles travel over the elevated concrete monolith hanging about 45ft (14m) above their plot every day on the main route between London and south Wales.

“I call it my free gazebo,” said Aimee. “I love it under here, when it’s raining you can walk to the shops without getting your hair wet.”

The two-lane carriageway in the sky was one of the first sections of the M4 and a relic of the 1960s freeway building boom after the explosion in post-war car ownership.