Police have urged motorists not to panic after rocks and stones were thrown at several vehicles – including one police car – on the A27.
Windscreens were smashed and bonnets were damaged and Sussex Police believe a group of youths are behind the incidents.
Items were thrown from the Broad Road bridge, west of the Fishbourne roundabout, and from the side of the dual carriageway.
One driver managed to look up and spot a group of youngsters on the bridge.
Sergeant Anna Gent, from Sussex Police, said: "Hopefully kids have been talking about it and someone will have the courage to come forward.
"This is extremely dangerous and it is very fortunate that nothing more serious came of it.
"But this is an isolated incident and we would urge everyone not to panic. Words of advice need to be had."
There were numerous reports of items being thrown on the bridge and the dual carriageway along the stretch of the A27 west of the Fishbourne roundabout.
Police were called to the scene early on November 22 and searched the area but could not find the culprits.
Resident John Wingrove, who lives in Hambrook just two minutes away from the Broad Road bridge, said: "I am a driver and I use that bridge virtually every day.
"I am amazed and surprised something like this has happened. This is absolutely dangerous and they are putting people's lives at risk.
"In our village there are not many groups like that.
"We don't have gangs roaming around the neighbourhood. I can't imagine where they have come from."
Police are appealing for anyone with any information regarding these incidents to make contact with either PC Carwardine or PC Gilmour at Chichester Police station on 0845 60 70 999.
Alternatively, you can contact CrimeStoppers anonymously on 0800 555111.
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