A crane or digger being carried on the back of a truck is believed to have struck a bridge on the A12 causing the road to be closed.
The driver of the vehicle drove away from the scene near Kelvedon in Essex, heading towards Suffolk.
Police are now appealing for the driver to come forward and witnesses to contact Pc James Ireland at Braintree on 101.
A structural engineer has now declared the bridge safe, but repairs will have to take place.
Both lanes of the A12 were reopened by 1pm.
The A12 in Essex is closed in both directions after a vehicle struck a bridge.
There's long queues between J25 (Marks Tey) and J23 (Kelvedon South).
Have you ever been stuck behind two lorries on a dual carriageway as one slowly overtakes the other? It can be pretty frustrating. The idea of banning HGVs from overtaking has already been tried on the A14 and the M11.
Now Essex County Council wants a ban introduced on the A12. But lorry drivers say the idea will be dangerous and could prove expensive. Here's Tom Barton.
A12 Yarmouth Road southbound remains closed due to resurfacing work between Station Road and B1385 Corton Road.
A12 Yarmouth Road Southbound closed due to resurfacing work between Station Road and B1385 Corton Road. It's been closed since Saturday afternoon when a large hole appeared in the road near to Denes High School. Diversion via A1117 Millennium Way and A1133 Normanston Drive / St Peter's Street.
The A12 Yarmouth Road in Lowestoft, Suffolk, is closed southbound for emergency repairs
Traffic building up on A12 London bound around J12 A1023 Chelmsford Road / B1002 (Mountnessing / Shenfield).
One lane is blocked due to an accident on the A12 London bound at J14 Wantz Road / B1002 Roman Road (Margaretting), congestion on A12 to J16 B1007 Stock Road (Galleywood).
There are long delays and one lane is closed due to an earlier accident on the A12 London-bound before the A120 (Marks Tey), but congestion is back to the Colchester Bypass.
Lane one remains closed, with the vehicles now on the hard shoulder. There has been a second accident in the queues near J26 Eight Ash Green, blocking the outside lane.