Youngsters have been warned they are putting lives at risk on Wearside's roads.
Youths have been throwing objects from bridges in Washington.
Police were informed an object had been thrown from a bridge on the A19 yesterday. The object landed on the carriageway and motorists were able to avoid it, but it caused considerable danger to road users.
A number of other reports have been received. The roads affected are the A19 and A1231.
Witnesses with information about those responsible are urged to come forward.
Acting Neighbourhood Inspector Les Goodliff said: "Throwing objects onto busy roads is reckless and incredibly dangerous. The objects do not need to hit a vehicle to cause damage, the very shock of an item hitting the carriageway could cause a motorist to lose control of their vehicle and cause a collision.
"Youths may think it is a bit of fun but they need to be aware of the danger they are causing, we are carrying out enquiries to trace those responsible and we will take action against them - if it's found their actions were directly responsible for causing a serious collision then the consequences will be far worse.
"We'd also like to ask parents to reinforce this message and remind their children of these dangers.
"If anyone has any information that can help in our enquiries then we would encourage them to come forward and pass this on to us.
Anyone with information should contact Northumbria Police on 101 quoting reference 835 250216 or Crimestoppers anonymously on 0800 555 111.