Police are treating the incident as attempted murder.
A woman suffered severe rib and facial injuries when a block dropped from West Hanningfield bridge smashed through the windscreen of the car she was in. The driver was less severely injured in the incident.
The accident happened at about 10pm on Thursday evening but about an hour earlier a similar incident had occurred when a concrete block was dropped from Fryerning Lane bridge on to another car.
The driver and passenger were not injured but were said to be severely shaken.
A spokesman for Essex Police said: “Detectives are linking both incidents and treating both as attempted murder. Both bridges have been taped off while forensic investigations continue.”
