Frances Mullen was driving home from work when her windscreen was shattered by the ice ball. Luckily, she managed to pull over and escaped injury but police have warned it could have been much worse.
Officers say it is just the latest in a spate of ice-throwing attacks on both motorists and pedestrians.
Ms Mullen, 39, said she initially thought the ice had fallen from a lorry in front of her as she drove home along the A899 at Livingston. She said: "Just as we approached the overhead bridge I noticed a spray of snow coming from the top end of the trailer and within seconds this big lump of ice totally obliterated the window.
"I pulled off the road and then a lorry pulled up in front of me and the driver said he'd seen kids up on the bridge when it happened."
Ms Mullen, a bus driver, said it was only when she got back to her home in Bathgate that she thought about how much danger she had been in.
She said: "I wasn't too bad at the time, but after I was in an awful state, I couldn't stop shaking because the shock had set in. If it had shattered through the window and hit me I'd have been off the road. It was like a massive hammer hitting the windscreen."
Police say the attack at 7.45pm on Monday was the second time youths had been reported dropping ice from the bridge at the Houstoun interchange, and one of nine reports of young people throwing ice at pedestrians and drivers in West Lothian over the past week.
On Saturday evening a taxi driver in Livingston called police after his vehicle was pelted with ice balls by a gang of a dozen youths.
Inspector Stephen Elliot, of West Lothian Community Safety Unit, said: "This behaviour is extremely reckless and dangerous. The victim was unhurt, but she was very shaken by the whole experience and it's just fortunate that she wasn't injured, and she didn't crash her car and cause an accident with other drivers.
"They're indiscriminate attacks. I think it's young people doing what young people do, but they don't realise the possible consequences."
West Lothian executive councillor for services to the community, Frank Anderson, said: "If a bus or car were struck by a piece of ice, it could be catastrophic if a driver had to swerve or lost control.
"Potentially, someone on the receiving end of this reckless behaviour could be seriously injured and those responsible would be held accountable."