The 54-year-old driver was unhurt but shaken when her black Hyundai's sunroof was smashed in the incident.
She was travelling along the A27 eastbound yesterday at 4pm when the item was thrown from the Peronne Road bridge, which crosses the M27 and A27 between Hilsea and Cosham.
Roads policing officers now want to speak to any witnesses and the youths seen standing on the bridge at the time.
PC Sarah McEwen, from the Havant Road Policing Unit, said: 'We would like to speak to anyone who knows the circumstances surrounding this incident which caused £500 worth of damage to a car.
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'While this is a rare occurrence, this mindless act of criminal damage could have led to the driver sustaining serious injuries, or resulted in a serious multi vehicle accident.
'Due to the time of day, there would have been a number of people crossing the bridge who may have witnessed this incident and may be able to assist police in their enquiries to identify the offenders.'
Call PC McEwen on 101, quoting 44170030067, or call Crimestoppers anonymously on 0800 555 111.