Drivers on the M1 in Hertfordshire continue to face long delays after a slip road was blocked by an overturned lorry during the morning rush hour.
The lorry also spilled fuel and has led to the closure of junction 6a, the exit slip road for the M25, causing long traffic queues southbound.
The lorry driver suffered severe head injuries and has been taken to a hospital in London, police said.
Congestion began to build at about 0800 GMT and went back to junction 9.
The incident was classed as severe by BBC Travel at 1215 GMT.
Diversions have been put in place and recovery of the lorry is being carried out.
Police said: "A lorry driver, for reasons as yet unknown, lost control of the vehicle.
"The lorry clipped the barrier, causing the vehicle to overturn. The driver, a man, suffered a serious head injury."
Police are appealing for any witnesses who saw the accident or saw the vehicle prior to the collision to contact police.