Police are urging drivers to be vigilant after reports of objects being thrown at vehicles on busy roads.
The warning comes after an unknown object is believed to have been thrown from a flyover and landed on a white Ford Transit van on the A47 at around 8am on Monday.
The driver had been travelling between the Pullover and Hardwick roundabouts in King’s Lynn, when the incident happened.
It left the van’s windscreen extensively damaged.
PC Amy Laws, said: “If the victim had not acted so swiftly, or if the roads were busier, the outcome could have been a lot worse.”
The incident follows three earlier cases, in which rocks the size of golf balls were thrown at vehicles on the A17, near to Hay Green Road North, Terrington St Clement, on July 5 and 19 and September 6.
PC Laws added: “I would like to warn motorists about this worrying trend and ask them to contact us immediately on 999 if they see someone acting suspiciously near to a flyover or the side of the road.”
Anyone with information shoould contact PC Amy Laws at King’s Lynn police station on 101.