Police have issued a stern warning of the possible consequences of throwing objects from road after the incident on Saturday.
The driver, a women in her 20s, had reached Junction 3 on the London bound carriageway at about 6.45pm when the object hit the windscreen of the car causing it to shatter.
Though the object caused damage to the windscreen of her Toyota Rav 4, she was unhurt.
In July 2010, 47-year-old Michael Baker of Littlehampton died from injuries sustained during
a collision involving an articulated lorry and a car recovery pick up truck on the A20 Swanley
interchange, after a rock was thrown at the lorry. Investigations continue to find those
responsible. These incidents are not believed to be connected.
Inspector Nick Sparkes said: "This could have quite easily smashed the windscreen and
struck the driver with disastrous consequences. There have been incidents of this kind in before and I cannot emphasise strongly enough that on this occasion the driver was lucky. "Throwing anything at a moving vehicle is irresponsible and malicious, but to throw something large from a bridge at a moving vehicle could kill someone. I encourage anyone with information about this to contact the police.”
Witnesses are asked to call Kent police on 101 or Kent Crimestoppers anonymously on 0800 555 111.