Wales' Christmas getaway has begun with travel experts warning people to set off early.
Wednesday to Friday were forecast to be the busiest days on the roads as motorists bid to beat a weekend rush, traffic analysts said.
Cardiff Airport was also expecting to handle 30,000 passengers in their busiest holiday season for seven years.
Rail passengers to and from London were warned Paddington station will be shut on Christmas Eve and 27 December.
More than 20 million vehicles were predicted to hit the UK's roads across the festive season and the RAC said the worst delays are expected to be on Friday evening.
Friday commuters will be joined by 1.25 million motorists visiting friends and family, with the worst hold-ups expected between 16:00 GMT and 20:00.
Inrix traffic analysts said motorists should expect hold-ups on the M4 motorway at Newport near the Brynglas Tunnels and around Swansea.
In north Wales, congestion on the A55 has been predicted to be highly likely around the Britannia Bridge, linking Anglesey and the mainland, because of increased demand on Holyhead ferry port.
"With a mixture of commuters, people driving to see friends and family, last-minute shoppers and online delivery drivers, Friday evening is expected to be the busiest on Welsh roads, " said Dr Graham Cookson, Inrix's chief economist.
"Drivers would be well advised to take alternative routes or avoid driving during peak times altogether."
National Express will also run coaches from south Wales on Christmas Day as part of its largest festive schedule.
Trains will not run across the network on Christmas Day and Boxing Day while Great Western Railway's (GWR) south Wales mainline services to and from London will either start at London Marylebone, Slough or Reading on the days either side because of the £40m upgrade work.
GWR told passengers its limited services from Marylebone will be in high demand.
Operations Manager Rob Mullen said: "We advise customers to plan their journey in advance and travel on or before 23 December if possible."
However, a Virgin Trains strike on north Wales services on Friday has been called off.
Holyhead, Aberystwyth and Pwllheli services to and from Birmingham will terminate and begin at Wolverhampton between Christmas Eve and 2 January because of track upgrade work.
Network Rail was not planning engineering works in Wales across the festive period but ATW said its services will "start to shut down" at 20:00 GMT on both Christmas and New Year's Eve.
Journey times were expected to be longer on services between south Wales and Portsmouth from 24 December to 1 January due to engineering works.
Cardiff Airport expected to handle 400 planes - 200 arriving and departing - between Friday and 2 January with Amsterdam, Dublin and Edinburgh the top festive destinations.
There will be additional frequency on Flybe services to Berlin, Cork, Dublin, Belfast and Jersey while new routes include Rome and Jamaica.
Both Stena Line and Irish Ferries were expecting their busiest time of the year sailing passengers between Wales and the Republic of Ireland.
Irish Ferries moved their 1,400 passenger, 700-car vessel, the Oscar Wilde, to the Holyhead and Dublin route, their busiest Irish Sea Service, to cope with "extra demand".