The Christmas getaway is under way with an estimated 1.3 million drivers taking to the roads on Thursday in a bid to avoid “Frantic Friday”.
Motorists travelling to spend the festive period with loved ones were warned to expect long delays.
The worst jams on Thursday will be on the M6 southbound between Merseyside and Staffordshire, with hold ups predicted to reach four hours and 21 minutes, according to traffic information supplier Inrix.
Other hotspots include the M3 westbound in Surrey and the M74 southbound from Glasgow to Hamilton.
Journeys are set to be even slower on Friday as the last of the year’s commuter traffic clashes with the Christmas getaway, amid a rail strike.
The RAC expects traffic to peak between 4pm and 8pm on Friday.
Inrix data shows that the worst non-accident traffic blackspot on the Friday before Christmas 2016 was the A303 at Stonehenge.
There were tailbacks for seven miles when congestion at the notorious bottleneck peaked shortly after 6pm on December 23 last year.
Inrix chief economist Dr Graham Cookson said: “Drivers would be well advised to take alternative routes or avoid driving during peak times altogether.”
RAC traffic spokesman Rod Dennis added: “Things tend to get worse on the last working day before Christmas, with ‘Frantic Friday’ this year falling on the 22 December.
“We strongly urge drivers planning long journeys to avoid this day if they possibly can.”
The RAC estimates that 11.5 million leisure trips will be made by car between December 17 and Christmas Eve, but this will be eclipsed by the 17.5 million taking place between Christmas Day and New Year’s Day as people take advantage of post-Christmas sales at shops.
Almost 400 miles of roadworks will be lifted by Highways England to ease journeys.
The “roadworks embargo” will be in place on motorways and major A roads from 6am on Friday until 12.01am on January 2.
A strike by rail workers on Virgin Trains on Friday will see a reduced timetable in operation, which could force more people to travel on already congested roads.
Some 260 engineering projects on the railway will also disrupt journeys.
Many of Britain’s mainline routes will be partially shut as Network Rail carries out its biggest ever Christmas investment programme.
Great Western Railway is urging passengers to complete journeys by Saturday “at the latest” as London Paddington will be closed between Christmas Eve and December 27.
The West Coast main line will be halted between Preston and Lancaster from Christmas Eve to December 27, with buses replacing trains.
Passengers travelling between London and Glasgow during this time are advised to go via Edinburgh, adding around an hour to journeys.
Southeastern, which serves London, Kent and East Sussex, will not be able to run trains to London Bridge, Charing Cross and Cannon Street between Saturday and January 1.
Network Rail chief executive Mark Carne said: “We know that our railway is up to 50% quieter than usual during the festive period so taking on and delivering these huge transformational schemes at this time of year minimises our impact on passengers.”
Coach operator National Express is running its largest ever Christmas schedule to meet increased demand from key locations such as Bournemouth, Cardiff, Leeds, Edinburgh, Portsmouth, Bristol, Glasgow and Manchester.
Airports, ports and international train stations will be exceptionally busy with more than 4.5 million people heading abroad over Christmas and the new year.
Travel trade organisation Abta advised people to leave extra time to catch their flight, boat or train.