Police are appealing for witnesses after two "bucket-sized" lumps of concrete were dropped from bridges onto cars, smashing their windscreens and injuring two people.
A 57-year-old woman suffered fractures to her face and ribs as well as internal injuries when a block burst through the windscreen, coming to a rest in the passenger footwell.
The incident happened at 10.05pm last night as the Nissan passed under West Hanningfield bridge on the A12 from Chelmsford to London.
Police are treating the incident as attempted murder. At a press conference today, Chief Inspector Nick Morris said: "It's only by pure luck a lady was not killed last night."
The woman had to be cut out of the car by emergency services. She was taken by ambulance to Broomfield Hospital, where she was in a serious but stable condition.
Her 56-year-old husband, who was driving at the time, suffered minor injuries. The couple were returning to their home in Harold Hill after visiting friends.
Forty minutes earlier, a similar sized concrete block smashed the windscreen and dented the bonnet of another car, a Vauxhall Astra, as it drove under the Fryerning Lane bridge on the A12 from London to Chelmsford.
The driver, a 26 year-old woman, and her 48-year-old mother were unharmed but badly shaken. They were returning to the Chelmsford area after a shopping trip.
Mr Morris said: "It could have been horrendous.
"It smashed the bonnet, smashed the windscreen of the vehicle. It's more luck than anything that no one was hurt."
Referring to both incidents, he said: "It is attempted murder. If you throw a large piece of concrete off a road bridge onto a large, busy road, you are likely to kill someone."
Mr Morris said the concrete block "might have been moulded inside a bucket."
He described it as "an item someone might place on a grass verge to stop someone parking there."
He added: "I don't understand why someone would throw it on a car. The only outcome is going to be someone getting seriously hurt."
Police are treating the two incidents as linked and suspect that more than one person may have been involved.
Mr Morris said: "I'm thinking there's a possibility that the person wasn't out there on their own."
He added: "I would have thought it's someone that knows the area or lives in the area."
Speculating on the age of those responsible, he said: "There's a minimum age someone has to be in order to drive between those two locations."
The bridge over the A12 on Brook Lane has been closed while police complete forensic examinations.
Police said there had been a number of occasions over the last few years where items had been dropped onto vehicles from bridges, but ruled out the possibility of installing CCTV.
Mr Morris said: "That would be an ideal solution to this, but everything comes down to cost. This is a rural location."
He said that the force will be "increasing our patrols for police covering this bridge".
Appealing for those responsible to come forward, he said: "I don't know the reasons; only they have chosen to do this. But they should come forward to take responsibility for their actions."
He added: "I'd like to ask anyone that saw any suspicious activity at the road bridges last night, particularly in the Chelmsford area, to come forward."