A woman and her child were lucky to escape unscathed when an object was hurled at their car, causing a rear window to shatter.
The unknown object was thrown at the moving vehicle as the mum and youngster travelled along Cambrian Way in Highfield, Hemel Hempstead, at around 6pm on Friday.
It caused the window to crack and when the driver stopped her car the glass completely shattered.
Herts Police spokesman Sarah Spence said: “Thankfully no one was injured on this occasion. However, incidents of this nature can have extremely serious consequences. In some cases elsewhere in the country people have been seriously injured.
“This behaviour is not acceptable and we would urge anyone who has any information about who is responsible to contact police.
“We would also like to ask parents to remind their children of the dangers of throwing stones at vehicles and the fact that it could end with a criminal record.”
The victim saw several youths running away from the scene.
Investigating officer PC Michael Ball said: “This is a serious offence and I would urge anyone with any information about what happened to call the police.”
He can be contacted by calling 101 and quoting crime reference number D1/13/750. Alternatively call Crimestoppers anonymously on 0800 555 111.