Police say there could have been "fatal consequences" after a stone was thought to have been thrown at a moving car from a footbridge over a busy road.
Police were called to a stretch of the A38 between the Manadon roundabout and the Leigham junction in Plymouth at about 4pm on Friday.
Driver Brian Dix, aged 50, from Whitleigh in Plymouth, and his wife were in their blue Vauxhall Corsa travelling eastwards under a footpath. Mr Dix said: "I just remember glimpsing some kid on the bridge, then it was like a shotgun blast as something hit the driver's side of the windscreen.
"It freaked me out, but I kept the car under control and pulled on to the grass verge.
"It was terrifying. There were shards of glass all over our clothes.
"What mindless idiots. The police said there was no way it was lifted up by another vehicle. Luckily, we were only doing around 50 miles an hour.
"If I was in my last car, a Honda Prelude 2.3i, I would've been doing about 70mph and it would've been much, much worse.
"The police were here straight away and they saw a young boy crossing on the bridge who couldn't keep from looking at us.
"I've had an officer call me about the incident and they said there had been a spate of these incidents happening of late. If the stone had been any bigger or I had lost control, it could've been horrendous."
A police spokesman said: "While nobody saw a stone being thrown, the damage was too big to have been caused by a stone being thrown up by other vehicles on the carriageway and there were no other vehicles nearby.
"They have had an amazingly lucky escape. We could easily have seen fatal consequences."