Bank holiday revellers have been urged to drive “after 9pm” as motorists are warned to expect heavy traffic on one of the busiest travel days of the year.
Drivers have been advised to delay bank holiday getaways till the evening tonight, to avoid severe queues on popular tourist routes.
There will also be severe disruption on parts of the railway network this weekend as Network Rail carries out hundreds of engineering projects.
Tourism bosses are anticipating the number of Britons making the most of the August bank holiday by taking a domestic short break or holiday will be the highest on record.
Research by the RAC and traffic information supplier Inrix suggests 16.5 million leisure journeys will take place on UK roads between Friday and Monday.
Friday is expected to be the worst day for jams as five million drivers embarking on journeys for a holiday or day trip battle for road space with commuters.
Roads are likely to be busiest between 11am and 6.30pm.
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.
Routes to coastal resorts and countryside beauty spots are also likely to be among the busiest.
The best time to be on the road will be after 9pm, but that will be too late for millions of people travelling long distances.
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.
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"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."
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."
Highways England said it is keeping 97% of England's motorways and major A-roads free from roadworks this weekend.
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 being closed as part of a major project which will prevent any trains from running between London and Peterborough.
Track replacement work near Macclesfield and Milton Keynes will reduce the number of trains departing and arriving at London Euston via the West Coast Main Line.
Major upgrade work is taking place in Newark, Nottinghamshire, affecting East Midlands Railway's Lincoln services to and from Leicester and Nottingham.
Network Rail is carrying out 448 engineering projects over the weekend.
The Government-owned company said it carries out significant projects over bank holidays as "far fewer people" use the railway at those times compared with a normal working day.