Drivers are being warned they could face long delays on major routes over the August bank holiday period, while train journeys will be impacted by rail engineering works.
Issuing the alert, the RAC urged motorists to set off as early as possible or "be prepared to spend longer in traffic".
The RAC said some three million journeys for holidays or day trips are expected to be made on Friday, rising to 3.4 million on Saturday, 2.4 million on Sunday and 2.7 million on Monday, with an additional 6.1 million drivers planning a leisure trip at some point between Friday and Monday.
With over 17.6 million getaway trips by car expected, these are the roads with the worst-expected congestion, as well as the train lines that will be closed.
Which roads are likely to be the worst affected?
Transport analytics company Inrix has predicted the M5 between Bristol and Devon will have the most severe getaway traffic.
It said the stretch from junction 15 north of Bristol to junction 23 for Bridgwater is likely to see delays of more than 40 minutes on Friday and Saturday.
Delays exceeding half an hour are forecast on Friday on the M20 in Kent, as the route is taken by a large proportion of vehicles making Channel crossings via Dover or Folkestone.
The warning covers journeys from junction seven near Maidstone to junction three (Addington Interchange), as well as from junction one at Swanley to junction five at Aylesford.
Major rail lines closed
Rail passengers are being warned that some major routes will be closed for engineering work, as Network Rail undertakes 261 projects across Britain.
There will be no long-distance services between London King's Cross and Peterborough on Saturday, which will disrupt Anglo-Scottish journeys by LNER and Lumo on the East Coast Main Line.
Avanti West Coast will operate a reduced and amended service to and from London Euston.
No services will operate between Birmingham New Street and Birmingham International between Saturday and Monday.
Affected Avanti West Coast and CrossCountry services will be diverted, increasing journey times.
London Northwestern services will run to and from Birmingham International only.
(c) Sky News 2025: Revealed: The roads set to face worst bank holiday traffic
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