Highways England closed lanes three and four of the southbound carriageway at around 5.50pm on Thursday (August 8) due to a collision between junction 31A for Longridge and junction 31 for Samlesbury.
The incident, believed to involve a motorbike, was expected to clear between 8.45pm and 9pm, according to Traffic England, with normal traffic conditions expected by 10pm.
There were also severe delays on to the M55, with traffic heavy from the junction one Broughton Interchange and queues back towards the junction three Wesham Interchange.
On the M6, traffic was backed up as far as junction 33 for Lancaster South.
Delays of up to 50 minutes were also reported.
At around 7.30pm, all lanes were reopened.
A Highways England spokesman said: "Delays of around 30 minutes above normal travel time remain on approach, though these are now expected to ease."