A fourth lane is due to be added to the hill by 2006

The pilot scheme aims to ease jams on a two-mile stretch of the motorway in North Somerset during the holidays.

Vehicles towing trailers and caravans are asked to stay in the inside lane up Naish Hill, near the Avonmouth Bridge.

Drivers are asked to comply from 1600 to 2000 on Fridays and from 0800 to 1400 on Saturdays until August 28. The Highways Agency will assess the scheme.

Severe tailbacks

Southbound, Naish Hill is a difficult climb for motorists pulling a caravan and slow vehicles often cause tailbacks, making it an unpopular stretch for other road users.

Towing drivers are also being encouraged to vary journey times and avoid travelling in rush hour.

If I see another caravan I'll burst. With a few gracious exceptions, they are a nuisance to other road users, driven with little consideration Michael Wells, UK Have your say

Highways Agency area manager Malcolm Wilkinson says the two miles up Naish Hill are particularly problematic.

"Slow-moving vehicles pulling out to overtake even slower-moving vehicles on the inside lane results in all remaining traffic trying to use the outside lane, causing severe tailbacks".

If judged a success the scheme will run again at holiday weekends next year.

In the longer term, there are plans to add a fourth, "climbing" lane on Naish Hill.

Work is due to start in November 2005 and should be completed by summer 2006.