An HGV had its windscreen smashed by two teenage boys seen throwing items off a bridge over the A505 Baldock Bypass.
The incident occurred on 25 August at around 2.45pm and caused so much damage the driver of the artic was unable to continue his journey.
Police are now hunting the youths that threw the missiles.
PC Zak Watson of Hertfordshire police said: “This is completely reckless behaviour and could have caused a collision to occur resulting in serious injuries or worse.
“I’m appealing to anyone who was driving in the area around the time of the incident who may have seen those involved on the bridge or may have dash cam footage that could identify them to please get in touch.”
PC Watson added: “The offenders were standing on the A507 Clothall Road bridge at the time, so if you were travelling on this road and saw them or have dash cam footage please also contact me.
“It is important that we identify those involved so that we can ensure there are no further incidents of this nature.”
Hertfordshire police said one of the boys was wearing a hoodie and dark trousers while the other was wearing a white hoodie.
Anyone with information can email PC Watson at [email protected], report information online at herts.police.uk/report, talk to a force communication room operator via web chat at herts.police.uk/contact, or call the non-emergency number 101 quoting crime reference 41/65738/21.