Drivers are being warned to expect lengthy traffic jams on major routes in the run-up to Christmas.
The busiest days for getaway travel will be December 23 and Christmas Eve – which will both see nearly 17 million cars on UK roads, the AA estimated.
A survey of more than 12,000 motorists indicated that 51% plan to make a car journey on December 23, with 50% expecting to make a trip the following day.
Potential congestion hotspots identified by the AA include the M25, the M5 between Bristol and Weston-super-Mare, and the M6 around Birmingham.
Long delays are also anticipated on the M1 from Luton northwards, the M60 and M62 in north-west England, the M4 which runs between west London and south-west Wales, and the M27 in Hampshire.
December 17 is expected to be the busiest day for High Streets, retail outlets and shopping centres as 20% of respondents to the survey said they will go Christmas shopping on that day.
Business-related travel is expected to drop off after December 16 until the new year.
AA president Edmund King said: “While December 23 and Christmas Eve look set to be the busiest travelling days, planned industrial action by rail staff may lead to increased levels of cars on our roads.
“Our expert patrols will be working throughout the holidays to help fix cars that suffer problems, while providing assistance to drivers should they be involved in a crash.
“Many breakdowns are preventable, so checking your vehicle before you set off is very important.”
He urged drivers to examine levels of fuel, oil, coolant and screen wash, and ensure tyres are correctly inflated.