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.
Heavy downpours across England are causing major disruption for road and rail users.
Network Rail said some areas had seen two months’ worth of rainfall in one day with drains overwhelmed.
Rail operator Southern has advised people to avoid travelling, delay journeys or use alternative routes.
A total of 31 flood alerts are in place across the country, with some areas set to see up to 60mm of rain, particularly over the first half of the day.
Southern tweeted “train services running across the whole network will be cancelled or delayed”.
A Network Rail spokesman said engineers were “out in force” pumping water away from areas.
“Across the south east, we suffered over a month’s worth of rainfall in just one day and in some areas the downpours equated to two month’s rain,” he said.
“We’ll continue working to keep passengers moving and then we’ll review the drainage systems which have suffered problems to see if we can do anymore to avoid similar incidents in the future.”
Leicestershire Police said it was receiving a “high level” of calls about blocked roads, while a driver narrowly escaped injury after a tree fell on his car in Thurnby.
A woman in her 80s and her dog were rescued from a flooded property in West Kingsdown, near Sevenoaks in the early hours.
Kent Fire and Rescue Service said she had been trapped in her home up to waist height.
The M25 was closed in both directions for almost eight hours after two sinkholes were discovered following a crash at about 23:30 BST, and in Devon, a thatched house caught fire when it was struck by lightning overnight. No-one was injured in the blaze.
Rain has also hit the Cricket World Cup for the second day in a row as Bangladesh’s game against Sri Lanka in Bristol has been delayed.
BBC Test Match Special’s Simon Mann said: “It’s a grim scene. Play today is extremely unlikely.”
Met Office meteorologist Alex Burkill said the UK was in for “some treacherous weather”.
Weather warnings are expected to remain in place for much of the day, with forecasters predicting parts of the UK could be inundated during the rest of the week.
North-eastern parts of England and the Midlands are expected to bear the brunt of the downpours on Tuesday, with a yellow warning in place until midnight.
The Environment Agency has issued three flood warnings in south-east England, for the River Wandle at Morden, south London, the Emm Brook at Wokingham and Ravensbourne at Bromley.
The agency said on its website: “River levels have risen as a result of localised thunderstorms. Flooding of property is expected imminently. Please take action.”
The Met Office said some parts of the country could see 60 to 80mm of rain on Wednesday and Thursday, possibly up to 100mm.
Mr Burkill described the figures as the “worst-case scenarios”.
“If you add it all up some places are likely to see over 100mm this week, which is around double the average they would get in the whole of June,” he added.
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