The great Christmas getaway began today with schools breaking up for the festive period.
The AA said it expected up to 18 million cars will take to the roads over the holiday period, with Thursday December 22 and Friday December 23 likely to be the busiest days.
Around 4.25 million people are travelling abroad for Christmas and the new year, while those staying at home will have to put up with engineering work on some rail routes.
A drivers' strike on London Underground is also likely to affect Boxing Day Tube services.
Wintry weather today generally led to wet, rather than snowy, conditions.
But the AA warned drivers to prepare for unsettled weather.
It predicted that getaway traffic in the run-up to Christmas was likely to be heavy on major routes, including the M1 in Buckinghamshire and Bedfordshire, the M4 between London and Newbury in Berkshire, the M25 around London, and the M4 and M5 in the West Country.
A recent AA/Populus survey of 16,647 AA members found that 68% were planning to stay at home this Christmas, with people in north-east England (78%) most likely to stay put and Londoners (60%) least likely to.
More than a quarter (26%) will be hitting the road to spend Christmas away from home, with 10% racking up more than 100 miles.
Keith Miller, AA patrolman of the year, said: "Although the current weather forecast for Christmas isn't as bad as last year, the outlook is still quite unsettled which means road conditions can be unpredictable.
"The Christmas period is always busy, so allow extra time for your journey and keep a close eye on weather and traffic reports before departing."
The 4.25 million figure for those Britons spending Christmas and the new year overseas covers the period from December 16 to January 3 and compares with just under four million for the festive period in 2010/11.
This year, around 1.7 million festive travellers will be leaving from Heathrow Airport, 425,000 from Stansted, 750,000 from Gatwick and 210,000 from Luton.
Also, 340,000 will be departing from Manchester Airport, 165,000 from Birmingham and 120,000 from Glasgow, while many thousands will be crossing the English Channel or travelling through the Channel Tunnel.
The passengers due to use Heathrow from December 16 to January 3 are expected to take 6,000 tonnes of luggage each day - equivalent to the weight of 20 jumbo jets.
Heathrow's most popular destinations this Christmas are New York, Dubai, Dublin, Amsterdam and Paris.
Travel organisation Abta said bookings to ski resorts in France, Austria, Switzerland and Italy had been going well.
Other popular destinations include the Canary Islands, Cuba and Mexico, while top city break destinations include Paris, Rome, Dublin, New York and Reykjavik in Iceland.
Those staying at home will find that buses are replacing trains on some rail routes. But rail chiefs have said that the number of replacement buses has halved compared with the 2010/11 festive period, and around 54,000 more trains will be running this year than last.
As much as £109 million of work is being done on the rail network over Christmas and the new year, with projects including work at Birmingham New Street station and at Reading station in Berkshire.
Travel will be free on Tubes and London buses from 11.45pm on New Year's Eve until 4.30am on New Year's Day.