Drivers are facing major tailbacks after an “unsafe bridge” shut the M6.
The motorway was closed between junctions 14 and 15 due to emergency bridge repairs at the A51, north of Stafford services.
Some panels urgently needed to be removed from underneath the bridge, although the agency said it was “structurally sound”.
The northbound carriageway reopened at 15:30 GMT after being closed for nearly 12 hours. Southbound remains closed.
The agency said: “During scheduled roadworks on the bridge which carries the A51 over the motorway we have identified that some fascia panels underneath the bridge that need to be removed.”
Operations manager Neil Taylor said: “Safety is our top priority and we have had to close the road to allow these panels to be removed.
“These panels, which are similar to fascia boards, are around 4ft long [1.2m] and weigh around 190kg, so we will need to use a crane to remove them.
“We are working as hard as possible to get these removed as soon as we can so we can reopen the road. In the meantime I would ask people to plan their journeys and allow themselves more time to get to their destination.”
Traffic is being diverted via the A34 and A500.
In a separate incident, the road was closed southbound between junction 3 (Nuneaton) and junction 2 (Coventry) for most of the morning.
West Midlands Ambulance Service said there was an accident at 08:30 GMT involving two lorries and two cars with a car driver being trapped in the wreckage.
The driver was released from the wreckage but was seriously injured.