Work is due to begin on Monday to create Britain's longest and most complicated contraflow system.
The contraflow comes into force between the Gordano Services area and Clevedon on the M5, for a total of nine miles.
It has taken six months of planning by engineers from the Highways Agency and will allow work to begin on building new crawler lanes on both carriageways.
The plan is to reduce congestion during peak holiday times. Motorists are warned speed cameras will operate.
A Highways Agency spokesman said the new lane were being introduced because heavy lorries and caravans currently crawl up the hills and cause long motorway tailbacks.
Work is expected to last for more than six months.