Drivers are being warned of “significant queues” as millions of people take to the roads for the start of the school holidays.
Congestion is expected to peak on Friday as holidaymakers share the roads with commuter traffic.
The RAC has warned of delays of up to 90 minutes on some roads over the weekend.
Traffic data company Inrix predicted the busiest period would be between 11:00 and 18:45 BST on Friday.
The RAC said its survey of 1,500 drivers suggested about 14 million journeys would be made for holidays and day trips between Thursday and Monday as many schools in England and Wales break up for the summer holidays.
Scotland and Northern Ireland’s schools have already broken up.
13:45 – 18:45Thursday
11:00 – 18:45 Friday
11:00 – 14:00Saturday
13:00 – 15:00Sunday
11:30 – 18:00Monday
Source: RAC/Inrix (times in BST)
Where will it be busiest?
Some of Friday’s longest delays are likely to include the M1 southbound between junction 16 in Northamptonshire and six in Hertfordshire, according to Inrix.
The company said journeys could take up to 90 minutes longer than usual.
It warned traffic on the M25 in Kent could see delays of about an hour between junctions four and one with long queues also possible on the M40 between Warwickshire and Oxfordshire and the M5 through Worcestershire.
On Saturday, the M20 near Maidstone is also predicted to see traffic slow to an average of less than 20mph between junctions seven and three.
Speeds as low as 24mph are likely on the A30 west between Pathfinder Village, Exeter and Whiddon Down, Okehampton, as holidaymakers head to the beaches of Devon and Cornwall.
Congestion will continue until Monday, with the M6 between Birmingham and Wolverhampton predicted to see speeds down to 17mph in the afternoon.
Delay forecasts are based on previous summer getaways as well as planned road closures and events taking place over the weekend.
Drivers were urged to check their oil, coolant and tyre tread and pressure to avoid breakdowns adding to delays.
Highways England’s head of road safety, Richard Leonard, said: “By doing the correct checks before setting off, motorists can make sure they keep everyone moving which means everyone can reach their destination safely and on time.”
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Where are the roadworks?
Roadworks taking place between Thursday and next Monday include:
- M6 southbound between J33 and J32 near Blackpool
- M6 northbound between J36 and J38 near Kendal, Cumbria
- M20 eastbound at Ashford, Kent, between J9 and J11
- M1 northbound entry slipway at J25 near East Midlands Airport
- M1 J13 to J12 near Luton.
Overnight roadworks include lane closures on the A30 eastbound exit to A3047 near Camborne, Cornwall from the Thursday to Saturday July between 20:00 and 06:00.