Ms Ansin quickly pulled over to the side of the road, and was joined by a car behind her which had ruined one of its wheels when it also hit the item.
The driver of the car told her that she had seen something drop down on to the motorway in front of them.
Ms Ansin said the incident had caused "considerable" damage to her car, which was towed from the side of the motorway and was now unable to move.
She said it could have been much worse, however, if the item had crashed through her convertible roof.
"What's scary is if that had fallen through my glass window or my roof.
"You just feel that they [over-bridges] should be more secure because it's a huge height for those sort of things to drop down."
A police spokeswoman said tonight the officer at the scene could not identify what the item was.
No one indicated to the officer at the time that something was thrown from the over-bridge either, she said.
