A pedestrian footbridge collapsed onto the road between junctions 3 and 4 of M20 motorway in Kent shortly after midday on Saturday.
Images on social media showed two lorries had been crushed under the bridge as it hung into the westbound carriageway.
In a statement Kent Police said they believed the bridge had collapsed as a result of being hit by a lorry.
South East Coast Ambulance service said that a motorcyclist in their fifties had been taken to hospital in Tunbridge Wells with suspected broken ribs as a result of the incident.
Kent Police confirmed that no other people were believed to have been trapped in the debris, and that the motorcyclist's injuries were not thought to be life-threatening.
Traffic was backed up for miles in both directions after the motorway – which runs between London and popular Kent tourist destinations including Margate, Whitstable and Canterbury – was closed.
The M20 is also the main route to the Channel Tunnel in Dover.
Bank holiday traffic was being diverted by police, but travel disruption is expected to be severe.