Bank Holiday motorists have been told to leave after 9pm tonight to avoid massive queues as millions of drivers hit the road for short breaks.
Major congestion is expected on routes to popular tourist destinations on the last long weekend before Christmas.
Network Rail is also carrying out hundreds of engineering projects which could lead to train travel misery.
As many as 16.5 million leisure journeys will take place between Friday and Monday, according to the RAC and traffic information supplier Inrix.
With the prospect of a heatwave and scorching temperatures as high as 33°C in southern England, even more people could make snap decisions to head for the coast.
Friday is due to be the worst day for traffic jams as five million drivers embark on a weekend trip.
The best time to drive will be after 9pm, while roads will be busiest between 11am and 6.30pm.
Tourism agency VisitEngland predicts 8.6 million Britons are planning an overnight UK trip this weekend, which is up from 7.3 million this time last year.
Trevor Reed, transportation analyst at Inrix, said: ‘Bank holidays have historically been one of the busiest times for road trips, and this year drivers could even see record-level travel delays.
‘Knowing when and where congestion will build can help drivers avoid the stress of sitting in traffic.’
RAC patrol of the year Ben Aldous said: ‘For many people, this coming weekend is the last chance to enjoy a long weekend this side of Christmas.
‘With the weather improving, that’s likely to mean a lot of cars on the road heading to popular tourist destinations.
‘A lot of the breakdowns we expect to see this weekend are preventable, so it’s vital drivers check over their car before setting out.’
The longest delays are predicted to be on the M25 anti-clockwise from Junction 4 at Orpington/Bromley to Junction 1 at Swanscombe/Dartford at around 3.15pm, with journeys taking 55 minutes longer than normal.
A northbound stretch of the M6 from Junction 18 at Northwich/Chester to Junction 24 at St Helens is estimated to have 54-minute hold-ups at around 2pm.
Network Rail is carrying out 448 engineering projects over the weekend, because it claims ‘far fewer people’ get trains over the Bank Holiday.
Major upgrade work is taking place in Newark, Nottinghamshire, affecting East Midlands Railway’s Lincoln services to and from Leicester and Nottingham.
Rail passengers are being urged to avoid the East Coast Main Line this weekend, with train operator London North Eastern Railway (LNER) issuing a ‘do not travel’ warning for the route between London and Scotland via York on Saturday and Sunday.
This is due to King’s Cross in London being closed as part of a major project which will prevent any trains from running between the capital and Peterborough.
Among the popular events on this weekend are Notting Hill Carnival in west London; Edinburgh Fringe festival; Reading and Leeds Festivals; Manchester Pride and Creamfields, near Warrington in Cheshire.