A driver who survived after her windscreen was smashed by a concrete block dropped from a bridge has spoken of her relief at the miracle escape.

Mother-of-two Lisa Horne, 26, said she believed that "somebody was looking over me" after she and her 48-year-old mother escaped unharmed when their Vauxhall Astra was targeted on the A12 in Essex on Thursday evening.

She told a news conference the attacker must be caught before someone is killed: "Obviously if I was going any faster, it may have been a different story. I do feel very lucky.

"It was just as if somebody had put their hands over my eyes, then I just heard a bang and my windscreen just shattered.

"I am scared to drive, I don't think I will be driving in the dark. I certainly won't be going down the A12," she said. "I am scared that I may never have come home to my children. I have got two young children. It has scared me a lot and it has made me realise how precious life is."

"I did swerve over to the other side of the road because I was travelling in the slow lane. I did go over to the other side of the road and mum shouted at me 'Lisa, get over into the hard shoulder'," she said. "I can't remember looking in my mirrors or actually moving over, so it was quite lucky really that there wasn't another vehicle as I went over."

Ms Horne's mother Stella told the news conference: "I remember seeing, like, a shadow coming down from the sky, and there was the biggest explosion. That was a rock hitting the car. At the time I thought it was something off an aeroplane, that was the first thought that was in my head."

She said she was "so proud" of her daughter and how she handled the incident. Asked what she thought of the perpetrator, she said: "I just don't know how they went home that night and slept, when they could have killed four people in half an hour. I can't sleep, thinking about it."

The incident happened under the Fryerning Lane Bridge near Ingatestone. Forty minutes later, a 57-year-old woman was left with fractures to her face and ribs and internal injuries when a "bucket-sized" piece of concrete was dropped on to her vehicle from a different bridge on the same stretch of road between Chelmsford and London.

Police investigating the attacks are trying to determine whether they have any definite links to more than 30 similar incidents on bridges along the A12 in Essex over the past three years.