Drivers are being warned of hour-long delays on some roads as traffic builds over the August bank holiday weekend.
Rush-hour motorists in much of the UK awoke to early morning fog, although conditions were expected to improve.
Congestion is expected to build between 11:00 BST and 18:30 as millions take to the roads.
Rail travellers have also been warned to expect delays, with the closure of London King’s Cross and reduced services on parts of the rail network.
The RAC said its survey of 1,900 drivers suggested about 16.5 million journeys would be made for holidays and day trips between Friday and Monday.
23 – 26 August
11:00 – 18:30 Friday
10:30 – 14:00 Saturday
12:30 – 14:00 Sunday
12:00 – 14:30 Monday
Source: RAC/Inrix (times in BST)
Where will it be busiest?
Traffic data company Inrix has said drivers should prepare for delays of about 55 minutes on Friday afternoon on the M25 anticlockwise between J4 at Bromley and J1 Swanscombe.
The M6 north between Chester and St Helens is expected to see similar delays.
Inrix said the M6 north between Cheshire and Liverpool would likely have 21-minute delays on Saturday afternoon while the M25 between Gatwick Airport and the M40 could have a 26-minute delay on Sunday afternoon.
On Monday the M6 southbound between Wigan and Stafford could have delays of an hour, with traffic particularly bad at about 14:15.
Delay forecasts are based on previous summer getaways as well as planned road closures and events taking place over the weekend.
Will there be roadworks?
Highways England said most work would have been completed or lifted with 97% of motorways “free from roadworks”.
But it said more than 50 sets of roadworks would stay in place.
A spokeswoman said: “To help people have smoother journeys we have removed as many roadworks as possible, leaving only essential work in place. However, it is not possible to remove all roadworks due to safety reasons.”
The longest sets of roadworks staying in place between Friday and Monday include:
- M4 J7 to J12 (near Wokingham and Reading) – 32 miles
- M6 J13 to J15 (near Stafford) – 18 miles
- M20 J2 to J7 (near Maidstone) – 16 miles
- M27 J4 to J11 (near Southampton) – 15 miles
- M1 J13 to J15 (near Milton Keynes) – 15 miles
- M6 J2 to J4 (near Birmingham – 14 miles
Is my train running?
Kings Cross station in London will be closed on Saturday and Sunday for planned engineering works.
No trains will run in either direction between the station and Peterborough or Cambridge.
A reduced timetable will be used on the West Coast Main Line because of engineering work near Milton Keynes, with two out of four lines closed.
Virgin Trains services will be down to one per hour on most routes and services between London Euston and Holyhead will run between Crewe and Holyhead only.
On Sunday there will be two trains per hour between London Euston and Manchester Piccadilly via Crewe and Wilmslow.
East Midlands Railway services will be replaced by buses between Nottingham and Lincoln and also between Newark North Gate and Lincoln on Saturday and Sunday.
Most LNER trains will be diverted between Doncaster and Peterborough. Replacement buses will run between Retford and Peterborough via Newark and Grantham.
All Grand Central services on Saturday and Sunday will be cancelled and Hull Trains services will be diverted to run to and from London St Pancras International.
More details can be seen on the National Rail Enquiries website.