Drivers returning to work after the holidays are warned to expect the start of two years of hold-ups as work on widening part of the M25 begins.
There will be a 40mph limit on the section during the roadworks
A 40mph speed limit will be imposed on one of the busiest stretches of motorway in the UK, to the west of London.
Roadworks start on Monday 5 January, the day many people go back to work after the Christmas and New Year break.
The work involves widening a seven-mile stretch between the junctions with the M3 and M4.
A slip road into the new Terminal 5 (T5) at Heathrow is also being constructed.
The motoring organisation AA said motorists would face narrower lanes, off-peak lane closures and a round-the-clock speed limit of 40 miles an hour while work takes place.
Stay safe: stick to the 40mph limit. There are 800 accidents, leading to 20 deaths and 1,300 injuries in roadworks every year
Plan routes: avoid the roadworks if possible. For example, drivers heading from Dartford to the M4 should consider going round the 'top' of the M25 instead of using the southern side
Time it right: build in plenty of extra time for trips involving the roadworks and surrounding routes.
Congestion charge: Remember too that the £5 road toll for central London applies from 0700, so motorists who normally time their journeys to get into London before the charge takes effect may need to set off earlier
Stay tuned: listen to travel reports on BBC London 94.9FM, or check the internet for the latest conditions.
The section is already the scene of delays with traffic often at a standstill.
AA Roadwatch manager Graham Webb said nearly 250,000 vehicles a day travelled on the section, more than twice the number it was designed for.
"Drivers heading into London from the west, or catching flights from Heathrow, should brace themselves for delays - particularly if the weather is bad," he said.
"But the widening work is, without question, long overdue."
The two years of delays are also likely to affect the M4, M3, A3, A30 and A308.
But Nic Ferriday, spokesperson for west London Friends of the Earth, said the widening was to do with the expansion of Heathrow.
"Before they came to power, Labour described the proposed widening of the M25 as 'lunacy'," he said.
"But when it came to power, it did not even wait for its own road review.
"Instead it succumbed to lobbying from British Airports Authority and decided to widen the M25 so that T5 could go ahead.
"And this was while the T5 Public Inquiry was still running, thereby proving that the government had already made up its mind on T5 and that the public inquiry was a charade."