The M6 motorway has been shut after part of a bridge fell onto the carriageway below.
It is understood a section of concrete fell from the structure and hit a car.
The incident occurred near junction 17 in Cheshire.
The bridge will now be inspected by structural engineers to ensure that it is safe before the motorway can be re-opened.
Southbound traffic between junctions 17 and 16 is being diverted onto A-roads.
North West Motorway Police tweeted: “The bridge deemed unsafe is Manor Rd and Reynolds Lane, which runs over the M6 near to Sandbach Services.”
A Highways Agency spokesman said: “A small piece of the bridge has been found to be structurally unsound over the southbound section of the motorway because some material car away from the bridge.”
He confirmed the southbound carriageway would not be re-opened until an inspection and any necessary repairs had been carried out.
The spokesman added: “We are trying to get it re-opened as quickly as we can.”
The bridge has been reopened following an inspection which cleared it as safe to re-open.
The Highways Agency will carry out an investigation to determine what caused pieces of concrete from the bridge to come loose.