Incidents occurred on the A12 in both the central and western divisions, which incorporate Chelmsford, Witham and Brentwood, on Wednesday, November 12.
Ch Insp Tom Diment, head of road policing, said: "Throwing anything at a moving vehicle is a very dangerous criminal act which could cause people to lose their lives.
"Anyone found to be throwing objects onto moving vehicles can expect to be arrested and assured we will use the full weight of the justice system against them."
Police are asking motorists to use extra vigilance. Anyone who sees anything suspicious on bridges is asked to call 999 immediately.