Police in Yeovil are warning motorists to beware, after objects were thrown onto cars from a bridge in the town.
Four vehicles have been hit by items dropped from the footbridge over the Queensway in the space of four days.
Three of the four had their windscreens smashed - one lorry's window was broken as it travelled at 30mph.
Inspector Mike Edwards said: "They may be doing this for fun but they have been very lucky that so far nobody has been seriously injured or killed."
He added: "What these people are doing is extremely dangerous and we have placed extra patrols in the area and our schools officers will be speaking to young people about the issue."
He appealed for anyone with information about the incidents, which happened between 4 and 8 December, to contact the police.