Motorists are being urged to delay bank holiday getaways until after 9pm on Friday 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 people making the most of the August bank holiday by taking a domestic short break or holiday will be the highest on record.
Today 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.
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.
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:
Track replacement work near Macclesfield will reduce the number of trains departing and arriving at London Euston via the West Coast Main Line.
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.
Among the popular events on this weekend are Manchester Pride and Creamfields, near Warrington in Cheshire.
VisitEngland director Patricia Yates said: