Drivers are being warned to prepare for long queues as millions of people embark on journeys to spend Christmas with friends and family.
The AA said Friday will be the busiest day on the roads this week, with an estimated 16.9 million journeys being made across the UK.
A further 16.6 million journeys are expected to be made on Christmas Eve.
Congestion could be increased due to a strike by thousands of members of the Rail, Maritime and Transport union (RMT) at Network Rail causing train services to finish at around 3pm on Saturday.
The RAC said roads will be busiest on Friday – the last working day before Christmas – between 10am and 7pm.
Transport analytics company Inrix expects journey times to be around 14% longer compared with the same period last year.
The RAC singled out the M25 clockwise between Junction 7 and Junction 16 as an area to avoid on Friday afternoon – with data from Inrix suggesting average vehicle speeds for this stretch of road could be as low as 26mph.
Other roads likely to be hit by congestion include the M60 near Manchester, the M6 in north-west England and the M40 in Oxfordshire.
RAC spokesman Rod Dennis said: “We’re expecting to see lengthy delays on parts of the M25 today as Christmas getaway and end-of-the-week commuter traffic combine.
“We advise drivers, if possible, to avoid these roads around this time or delay their trips until after 7pm this evening when traffic is predicted to be lighter.
“A single vehicle breakdown also has the potential to add to the queues, so we encourage motorists to complete a few pre-drive checks before setting out – in particular ensuring oil and coolant levels are correct, and tyres have plenty of tread and are properly inflated.”
National Highways said almost 98% of England’s motorways and major A-roads will be fully open until the end of January 2 due to it completing and lifting roadworks.
AA head of roads policy Jack Cousens said: “We are advising those heading out in their cars to be prepared for some congestion, especially on popular routes heading out of London.
“The rail strikes have convinced more people to travel by car this year, and while hundreds of miles of roadworks have been removed to ease the pain, it might not be enough to keep the queues away.”
Inrix transportation analyst Bob Pishue said: “With pre-pandemic levels of travellers hitting the road this holiday, drivers must be prepared for delays – especially in and around major cities.”