The accident happened on Saturday afternoon on the M876 motorway at Dennyloanhead, Falkirk.
The vehicle hit the pavement about 30ft below, rolled several times and ended up on its side.
Police and ambulance crews rushed to the scene but the driver was helped from the car with a chipped tooth and cut head and taken to a nearby house.
Several young boys had been playing below the motorway flyover.
"He seemed to just have had a cut on his head and a chipped tooth and was able to walk away"
One told the BBC Scotland news website: "We heard something and turned round and saw the car in mid-air - I'm sure it was upright flying through the air, it then came down and hit the kerb not far from us.
"It rolled several times across the road and came to rest on its side."
Another witness, Thomas Beresford, who is also a first-aider, came across the accident.
He said: "When I arrived I saw the car on its side and the driver signalling with his arm through the driver's window.
"By the time I got up there he was already out of the car, sitting on the wall. He seemed to just have had a cut on his head and a chipped tooth and was able to walk away."
Central Scotland Police said the 21-year-old driver was taken to Stirling Royal Infirmary but was described as "walking wounded".
A police spokesman said: "The incident happened on the Denny Road flyover when a vehicle landed on the road below, after rolling several times, we believe.
"The circumstances of how it ended up there are currently under investigation."
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