The M20 in Kent is closed between junctions 3 and 4, with Highways England saying a vehicle had struck a bridge on the eastbound carriageway.
One person, a motorcyclist, has been injured – but his injuries are not life-threatening.
Travel disruption is said to be severe, with the motorway expected to remain closed until at least Sunday.
Kent Police said the crash was being dealt with as a major incident but officers said no-one was believed to be trapped in the debris.
South East Coast Ambulance Service tweeted that a motorcyclist in his 50s had been taken by ambulance to Tunbridge Wells Hospital with suspected broken ribs after the bridge collapsed.
Image copyrightAPThe crash happened near the junction with the M26, which links the M25 London orbital with the M20.
The motorway is the main route to the Channel Tunnel and the Port of Dover for people travelling to France and across Europe.
The crash had knock-on effects across the motorway network after the eastbound M26 was closed to stop further congestion in the area, leading to queues on the M25, which travel reports said was “effectively down to two lanes”.
About three hours after the collision, all traffic trapped on the southbound M20 had been released, and on the northbound M20, traffic was queuing for four-and-a-half miles before the road closure.
Cross-Channel traffic was being diverted on to the A2 and M2 from the M25 and the Port of Dover, Highways England said.
Image copyrightFADE OGUNROSonny Gladdish was on the M20 when the crash happened.
He said a lorry carrying a digger was driving on the hard shoulder and made impact with the bridge.
Mr Gladdish said he heard the bang and was about four or five cars behind the collision when it happened.
He said the bridge collapsed on one lorry and he believed the motorcyclist had thrown himself off his bike to avoid the impact.
‘In pieces’
Witness Andy Sunnucks, 24, a graphic designer who was travelling from Maidstone to Sevenoaks, said: “We are about 15 cars back on the same carriageway as the collision… It looked like a lorry had jack-knifed, and I could see half the bridge was missing.
“We went to have a look and the back end of the lorry was in pieces.”
Alex T Magaisa, his wife and two young sons were driving to Gatwick Airport and were just passing the lorry when the bridge collapsed.
He said: “My instinct was just to drive through. My wife saw the bridge falling and there was a big noise. I had to manoeuvre through the debris.
“It was a big shock. It’s only just starting to sink in now what might have happened. We were right in the line of fire and we could have been crushed.”
Image copyright@EMMARAPHAELX/PAAnother witness, Sean Fisher, said: “I spoke to [the woman in] the first car behind the lorry. She seemed to be all right. Her daughter was quite brave and went under the bridge and was the first responder to the fallen motorcyclist.”
Some drivers also tried to stop traffic on the other side of the road, Mr Fisher said, as they feared the bridge might collapse on the coast-bound carriageway as well.
“At the moment it seems they are going to get the motorcycles out first, the wrong way on the road, and I’m not sure what they are going to do with the cars,” he added.
Broken ribs
Emergency crews were called to the scene on the London-bound carriageway at about 12:10 BST.
A spokeswoman for Kent Police said: “It is believed a lorry collided with a pedestrian bridge which has collapsed onto the carriageway below.”
She added: “No people are believed to be trapped in the debris, however one person is believed to have suffered injuries, not reported to be life-threatening at this time.”
Image copyrightFAYE OGUNROPeople took to social media to share pictures of the collapsed structure.
Helen Ayres posted: “Now that’s what I call timing. Just as well the M20 is usually free-flowing on a Bank Holid… Oh. ”
Jimmy tweeted: “Couple of guys playing football on the m20; not something you see everyday #m20 #motorway #traffic …”
Some drivers objected to the handling of the aftermath of the crash, with William Gaunt tweeting: “@kent_police why so long to close M20? 1000s joined 45 mins after bridge strike now trapped.”
Image copyright@EMMARAPHAELX/PAAn estimated 13 million drivers are expected to take to the road between Friday and Monday over the long weekend, according to the AA.
The busiest day was expected to be Saturday, with 10 million motorists expected to travel.
Image copyrightFADE OGUNRO[Source:-BBC]
