Millions of people embarking on bank holiday getaways are being warned over long delays on the roads and disruption to train services.
The RAC urged drivers to set off as early as possible or âbe prepared to spend longer in trafficâ.
Major roads in the South East and South West are expected to be among the most congested.
A strike on Saturday and Monday by workers at train operator CrossCounty who are members of the Rail, Maritime and Transport (RMT) union will affect services throughout the bank holiday period.
Elsewhere on the network, passengers are being warned some major routes will be closed for engineering work as Network Rail conducts 261 projects across Britain.
Monday is a bank holiday in England, Wales and Northern Ireland, and the last before the Christmas period.
The RAC expects 17.6 million getaway trips by car are expected to take place in the UK between Friday and Monday.
Three million journeys for holidays or day trips are expected to be made on Friday, rising to 3.4 million on Saturday.
Some 2.4 million leisure journeys are expected on Sunday and 2.7 million on Monday.
The RAC said an additional 6.1 million drivers are planning a trip at some point between Friday and Monday.
The figures are based on responses to a survey of 2,080 UK adults, extrapolated to the 34 million cars licensed in the UK.
Transport analytics company Inrix predicted the M5 between Bristol and Devon will have the most severe getaway traffic, with the stretch from junction 15 north of Bristol to junction 23 for Bridgwater likely to see delays of more than 40 minutes on Friday and Saturday.
Hold-ups exceeding half an hour are forecast on Friday on the M20 in Kent, which is a route taken by a large proportion of vehicles making Channel crossings via Dover or Folkestone.
The warning relates to journeys from junction seven near Maidstone to junction three (Addington Interchange), and from junction one at Swanley to junction five at Aylesford.
RAC mobile servicing and repairs team leader Nick Mullender said: âWith this bank holiday being the last opportunity to enjoy a long weekend before Christmas, our study shows a real eagerness to get out and about with nearly 18 million drivers planning getaway trips.
âMore traffic on the roads will inevitably lead to more vehicle breakdowns, especially if the sun makes an appearance and people decide on the day to visit popular destinations.
âWeâre expecting major roads to airports and coastal destinations to be extremely busy, especially the south-east and south-west regions which could end up bearing the brunt of most holiday hold-ups.
âAnyone planning routes through these areas should set off as early as possible or be prepared to spend longer in traffic.â
On the rail network, industrial action means CrossCountry will have no services on Saturday, significant disruption on Sunday and a reduced timetable on Monday.
Passengers are advised not to travel on Saturday and to travel on either side of the bank holiday weekend, or claim a full refund.
Union sources said the dispute centres on claims that CrossCountry has failed to honour agreements on staffing, safety and pay.
Shiona Rolfe, CrossCountry managing director, said the company is âcommitted to reaching an agreement with the RMT and remain available to continue talksâ.
Engineering work by Network Rail means there will be no long-distance services between London Kingâs Cross and Peterborough on Sunday, disrupting Anglo-Scottish journeys by LNER and Lumo on the East Coast Main Line.
Avanti West Coast will operate a reduced service to and from London Euston, with their trains between Birmingham New Street and Birmingham International being diverted, adding extra time to journeys.
CrossCountry services will also be diverted, extending journey times, while London Northwestern services will run to and from Birmingham International only.
Helen Hamlin, Network Railâs chief network operator, said the âvast majority of the railway will be runningâ but âworks on some parts of the network are unfortunately unavoidableâ.
