Two nine-year-old girls were arrested by police after they threw stones at traffic on a busy motorway.
The girls are too young to be held criminally responsible
The pair were filmed by CCTV cameras as they hurled rocks from a footbridge over the M25 near Merstham in Surrey.
One car had its windscreen smashed in the incident on 11 May.
At one stage in the footage, one of the girls is shown dumping dozens of stones she had collected and was holding in her jumper.
The film clearly shows the youngsters leaning over the footbridge and appearing to take aim at the cars underneath.
The youngsters were taken home by police but had to be released as they are too young to be prosecuted.
There is always a danger of copycat incidents but we must warn everybody of the tremendous dangers being caused to innocent motorists
Chief Inspector Mark Clark
Chief Inspector Mark Clark, from the Strategic Roads Unit, said: "This really has to stop now before more people are killed or maimed."
The number of cases where objects have been thrown into oncoming traffic from motorway footbridges has "dramatically increased" in the last few months, police said.
In the first quarter of this year Surrey Police dealt with an average of 26 incidents a month - twice as high as in any of the last three years.
Mr Clark said: "I appeal to parents of children who live within easy travelling distance of a motorway, please make sure your children are not participating in such a dangerous activity.
"There is always a danger of copycat incidents but we must warn everybody of the tremendous dangers being caused to innocent motorists.
"I warn both parents and their children who engage in this activity that prosecution for this very serious offence will always be our objective."
A spokeswoman for the AA said it was important to keep the figures in perspective because many more people were killed by speeding and drink-driving than by objects thrown at vehicles.
"We would like to see more CCTV cameras," she said.
"The fact that these two nine-year-old girls have been caught will work as a lesson or deterrent to those of any age contemplating doing the same."
Detectives investigating the murder of lorry driver Michael Little, killed two months ago by a brick thrown from a motorway bridge on the M3, say they now have a new lead in the case.