A minibus taxi driver was critically injured after he was flung from his vehicle, off a bridge and onto the M1 highway below, Johannesburg Emergency Services said.

"He was alone in the taxi at the time of the accident which occurred on the bridge above the M1 highway near Roodepoort," said spokesperson Nana Radebe.

She said the driver hit a trailer being pulled by a vehicle in front of him, lost control of the taxi, hit the barrier and was flung out onto the M1 north near the Crown interchange. He was not wearing his seatbelt.

"I am not sure how many metres he was flung... no one else was hurt in the other vehicle," Radebe said.

Traffic was severely backed up from the M1 off-ramp in Roodepoort.

"There is dead still traffic... I am not sure how far traffic is backed up."

He was taken to the Chris Hani Baragwanath Hospital with head and chest injuries.