This weekend will be the busiest summer getaway in at least eight years, drivers have been warned.
The RAC said an estimated 18.8 million leisure trips are planned in the UK between Friday and Monday.
That is the most since the company began tracking summer getaway numbers in 2014.
Leisure traffic volumes look set to peak on Saturday, a survey of 1,700 UK motorists suggested.
Transport analytics company Inrix believes the M25 – London’s orbital motorway – could see some of the worst jams, singling out the stretches between Bromley and the Dartford Crossing; Maple Cross and the M3; and the M23 to the M40.
The A303 near Stonehenge, Wiltshire, the M4 between Cardiff and Newport in south Wales, and the M5 south of Bristol are also likely to see queuing traffic at various points this weekend.
RAC spokesman Rod Dennis said: “The extreme heat that’s affecting much of the UK in the last few days might wane over the next few days, but we still expect the summer getaway to begin with a bang as a potentially record-breaking number of drivers take to the roads this coming weekend – and that’s despite the unbelievably high cost of fuel.
“With school terms in England and Wales finishing this week and millions of people ready for a well-earned break, we anticipate a frantic Friday followed by a woeful weekend on major roads across the country, with traffic and congestion likely peaking on Saturday.
“The advice to drivers heading off on a holiday by car is therefore clear – leave as early as you can in the morning or expect to be sat in some lengthy queues.”
Inrix transport analyst Bob Pishue said: “Drivers should expect traffic congestion to build throughout the day, especially on motorways.
“Despite high fuel prices, travellers do not appear to be giving up their road trips and holidays.”
National Highways head of road safety Jeremy Phillips said: “With schools breaking up for summer soon there will be more people on the roads and taking longer journeys.
“We know that breaking down can be a very upsetting experience, nobody wants to start off their holiday stranded at the side of the road, next to fast moving traffic.
“So we are reminding drivers to check their vehicles, particularly the tyres, before setting off.”