The �289m scheme aims to stops jams and improve safety

The work on one of the motorway's busiest stretches includes the creation of a car sharing lane, which will open after the UK's first, in Yorkshire.

A fourth lane will be built on 10 miles of the M1 between Watford and Luton.

Drivers face months of 40mph limits, lane closures, speed cameras and traffic jams as far south as London and as far north as Northampton.

The work, between junctions 6A and 10, will carry on until December 2008. About 160,000 vehicles a day, including those travelling to Luton airport, use this stretch of road.

Roads Minister Stephen Ladyman has said: "All users of the M1 will benefit from the extra lanes we're adding to this busy section of road".

The car sharing section will be between junction 7, near St Albans, and junction 10.

The carpool lane will only be open to vehicles with two or more occupants during peak hours but it has yet to be decided if motorcycles will be allowed to use it.

It is planned to widen the road as far as Milton Keynes later and the carpool lane could be extended to junction 13.