The M1 motorway was partially closed after a multi-vehicle collision.
The northbound carriageway was shut at about 11:50 BST between junction 9 for the A5183 at Redbourn and junction 11 for the A505.
The Highways Agency said it was due to a collision near junction 10 for Luton Airport. Bedfordshire Police said six vehicles were involved and three people suffered injuries.
The road reopened at about 15:15 BST but long delays remain in the area.
There are more than six miles of congestion back to junction 8 for Hemel Hempstead, which is expected to add about one hour on to normal journey times, the agency said.
The East of England Ambulance Service said two people were taken to Luton and Dunstable Hospital for further care.