Shoppers hoping for a Boxing Day bargain could face traffic jams as rail strikes see train services brought to a halt.
Thousands of people planning to travel by train have been forced to make alternative plans amid continued industrial action.
Hundreds of departures usually run on December 26 after the Christmas Day shutdown.
But Network Rail said Britain’s railways will remain closed for a second consecutive day due to a walkout by employees who are members of the Rail, Maritime and Transport union (RMT).
The AA said it expects 15.2 million cars on UK roads on Boxing Day, with shoppers and football fans among those likely to travel.
A spokesman said: “Traffic is likely to build around shopping centres as lots of people seek a bargain in the sales, meanwhile football fans will travel to see their teams.
“There is scope for localised traffic congestion and more short trips, but traffic should be dispersed throughout the day as people take their time after Christmas Day.”
New data has predicted that shoppers will spend 4% less this Boxing Day and during the post-Christmas sales due to the cost-of-living crisis.
Research by Barclaycard Payments found that the average shopper intends to buy £229 worth of items in the post-Christmas sales period, a reduction of £18 compared with 2021.
Passengers were warned about the impact of the rail strike a week ago – and heeded warnings appeared to lead to relatively smooth travel for most people on Christmas Eve when services finished up early.
Airport transfers such as the Heathrow Express and Stansted Express are usually among the most popular services on Boxing Day, but airline passengers will be forced to find other ways of getting to and from Britain’s airports.
A number of other planned services cannot take place due to the RMT strike.
Merseyrail was due to run a half-hourly service across its network in Merseyside.
ScotRail has historically operated a Boxing Day service in the Strathclyde area.
The operator also ran its first Boxing Day trains between Edinburgh Waverley and Glasgow Queen Street via Falkirk High in 2021.
Northern operated a service between Liverpool Lime Street and St Helens Central on Boxing Day last year.
Coach operators National Express and Megabus have experienced strong demand.
The RAC highlighted two stretches of the M25 as likely hotspots for queues.
They are from junction 7 to 16 clockwise, and from junction 4 to 1 anti-clockwise.