The bank holiday getaway has begun, with a weekend of frustration and delays forecast for those venturing on to Britain's railways and roads.
The warning follows the decision to carry out major work on four of Britain's busiest railway lines on the last public holiday of the summer.
Journeys on affected routes could take hours longer and motoring organisations say there could be an extra six million people on the roads as a result.
The expected traffic jams could be made worse by roadworks on motorways, including the M1 near Nottingham, the M5 and M6 near Birmingham, the M4 near Reading and the M11 and M25.
Worst affected on Friday night were the M4 westbound and M1 and M6 northbound, according to the RAC.
A 15-mile tailback between M4 junctions 23 and 32 was delaying drivers by 55 minutes, a spokeswoman said.
A 17-mile queue between M1 junctions 20 and 27 meant a 70-minute delay.
And M6 motorists were stuck in an 11-mile jam between junctions five and 10a.
The transport problems are likely to be bad news for those heading for the coast and countryside and major events including the Reading and Creamfields music festivals.
Defending the decision to allow the train line closures at one of the busiest times of the year, Transport Minister Kim Howells said the work would improve efficiency and save up to £600m.
Chairman of the Rail Passengers Council, Stuart Francis, said people had been left in the dark and would question whether the work had been properly planned.
The train routes affected by engineering works include the Great Western Main Line, which will be completely closed between Slough and Reading.
In East Anglia, the line from London to Stansted Airport is closed on Sunday and Monday.
Large-scale engineering works will see delays on the West Coast Main Line from Saturday 23 August until Sunday 31 August, as the line will be closed between Hemel Hempstead and Milton Keynes.
It will also be closed between Stockport and Manchester this weekend.
Passengers travelling between Birmingham and London have been advised to use the Chiltern Railways service, which is offering the only direct and frequent trains between the cities.
Delays are also to be expected on the East Coast line from London to Edinburgh.
A large number of smaller engineering projects will also be taking place, but operators are confident these will only cause minor delays.
BANK HOLIDAY WEEKEND EVENTS
Reading Festival - Friday, Saturday and Sunday
Leeds Festival - Friday, Saturday, Sunday
Creamfields Dance Festival, Liverpool - Saturday
Wales vs England, Millennium Stadium, Cardiff - Saturday
Notting Hill Carnival, West London - Sunday and Monday
British Superbike Championships, Louth, Lincolnshire - Monday
Network Rail chief executive John Armitt said carrying out the work over the weekend would cause less inconvenience doing it on a series of alternative dates.
This weekend alone 77 miles of track are expected to be replaced - a significant proportion of the 600 miles of line that need to be renewed every year.
Mr Armitt told BBC News: "The bank holidays are three-day weekends and when we are doing major works it gives us the opportunity to do these efficiently and at a reduced cost."
Keith Ludeman, chairman of the Association of Train Operating Companies, said the work had to be carried out on a weekend as twice as many people use the railways during the week.
But Richard Hope of the Railway Gazette magazine said: "The point of the railway is to serve the public, to keep the freight moving, to keep the passengers moving and that seems to be becoming a very low priority now."
The Automobile Association (AA) is predicting heavy congestion around popular destinations, and in areas hosting music festivals and sporting events.
The organisation has drafted in 100 extra patrols to assist with car troubles, and advises drivers to ensure tyres are well inflated and they have plenty of petrol and oil.
Routes out of London towards the West, South Wales, Devon and Cornwall will see extra congestion, as will the M2 and M20 to Dover and Folkestone.
Rugby fans travelling to watch Wales play England at Cardiff's Millennium Stadium on Saturday are also expected to add to normal traffic.
And the AA predicts extra congestion on Monday due to the British Superbike Championships in Louth, Lincolnshire.