Traffic has ground to a standstill on the A1 at Barnsdale Bar after a man fell from a bridge onto the carriageway.
Two dogs also fell from the same overbridge at Barnsdale Bar, Darrington, near Pontefract.
The man - who has not yet been named - and one of the dogs were both pronounced dead at the scene, confirmed a West Yorkshire Police spokesman.
The second dog is alive, he added.
Officers were called to reports of a ‘serious incident’ on the A1 shortly before 8pm.
The A1 was expected to remain closed southbound between Wentbridge and Barnsdale Bar until the early hours of Wednesday.
Hundreds of motorists were trapped by the closure.
Maitland Hyslop, 58, who was on his way to London from Hexham when he became caught up in the incident at 8.30pm, said he has counted around 20 police vehicles, a police helicopter and two ambulance vehicles at the scene.
He said police were attempting to get drivers to turn round on the closed southbound carriageway and exit the A1 in reverse.
Mr Hyslop said: “All the police are on the bridge. I am about 600 yards south of Selby service station.
“I’ve been told by officers they are considering trying to get cars - not HGVs - to turn about and go back the way they have come as the carriageway is closed to any oncoming traffic.”