The vehicle was one of three struck by debris which smashed the windscreens of two and dented the bonnet of the third.
Police have launched an appeal for witnesses to trace two people spotted on the bridge amid warnings a motorist could be killed.
Officers said at least one of the cars had young passengers inside when it was struck as it passed under the Lanark Road flyover near the Baberton turn-off.
An Audi A4 and Vauxhall Insignia were hit on the eastbound carriageway of the A720 at around 11.40pm on Tuesday, shattering their windscreens.
The cars were forced to pull over into the hard shoulder and were later escorted to Dreghorn services by police.
Thankfully, no-one was injured.
Driver Graham Burns was returning home to Dreghorn from a fishing trip when he heard a loud thud.
The window cleaner said: “It was late at night and the road was quite quiet at that time.
“I thought I had hit a bird or an animal and pulled up on the hard shoulder a couple of hundred yards away to check it.
“An object had obviously been thrown with some force. There was a dent about eight inches long on the front corner of the bonnet just above the headlight. I was doing about 60 or 70 miles per hour at the time. It could have been a lot worse.”
The 54-year-old also revealed that the incident was “preying on his mind” and that he had been avoiding the bypass until police catch the culprits.
“If I pass under a bridge I’m looking up to see if there is anybody on it,” he added.
Police are particularly keen to trace two males “acting suspiciously” near the flyover shortly beforehand.
Sergeant Neil Thomson, based at Wester Hailes police station, said: “These crimes demonstrate a complete disregard for the safety of others.
“Such recklessness is extremely dangerous and could have resulted in multiple serious injuries or worse – someone could have been killed.
“Our inquiries into this matter are very much ongoing and I am keen to trace two males who were seen in the area of the flyover on Tuesday night.”
The incidents were the worst problems suffered for some time involving debris being dropped on to the bypass.
Scottish Conservative shadow justice secretary Douglas Ross condemned the attack.
He said: “This behaviour directly threatens lives, of that there is no question.
“Throwing rocks at cars on one of Scotland’s busiest routes is irresponsible, dangerous and utterly criminal.
“I am sure the police will redouble efforts to catch these individuals and, when they do, they must be dealt with severely. This needs to be stamped out at once.
“It is pretty obvious that if this behaviour is not stopped one of these incidents will cause a major, life-threatening incident.”
Police can be contacted on 101 or via Crimestoppers on 0800 555 111.”