Drivers have been warned to expect delays on Thursday, with more vehicles predicted to be on the roads than any other day over the festive period.
The M4 around Newport, Swansea and Port Talbot is expected to be Wales' busiest road as the Christmas getaway begins.
Rail passengers travelling to and from south Wales will be affected by huge disruption over the festive period.
London Paddington closes on two days and buses replace trains on south Wales routes between Christmas and new year.
Delays of up to 20 minutes are predicted on parts of the M4 motorway as traffic experts warn "pre-Christmas panic" on the road will be at its height on Thursday.
Data analysts at Inrix believe the worst stretches of road in Wales will be:
- The M4 westbound around Newport - between the Prince of Wales Bridge and Brynglas Tunnels - at 16:30 GMT, with delays of 20 minutes expected
- The M4 westbound around Port Talbot and Swansea - between Margam and Llangyfelach - at 08:30, with delays of 13 minutes expected
- The M4 eastbound around Newport - between St Mellons and Celtic Manor - at noon on 28 December, with delays of 17 minutes expected.
"Our figures suggest many more drivers are planning leisure trips by car in the run-up to Christmas this year," said Ben Aldous, of the RAC.
"So bumper-to-bumper traffic on some motorways and major A-roads is going to be near-guaranteed."
15:00 - 19:00Thursday
12:00 - 19:00Friday
10:00 - 15:00Saturday
10:00 - 16:00Sunday
10:00 - 15:00Monday
14:00 - 18:00Tuesday
Inrix predicts there will be 31 million trips by car over Christmas with Thursday being the busiest as people "compete with commuters and shoppers for road space".
Rail bosses have urged passengers over Christmas and new year to check their travel plans due to the amount of disruption to Welsh services.
Paddington is shut on Christmas Eve and 27 December, while replacement buses will run between Bristol Parkway, Chepstow, Cwmbran, Ebbw Vale Town and Cardiff Central from 27 December to New Year's Day.
- Christmas Eve to 27 December: GWR services between south Wales and London start and end at Reading because of engineering work near Paddington station. Buses will replace trains between London and Reading
- 27 December to 1 January: All trains in and around south Wales will be affected by engineering work around Newport and Cardiff. It will affect GWR London and Portsmouth routes, Cross Country services to Nottingham and all Transport for Wales trains in south Wales.
Rail passengers have been warned to expect train delays and cancellations on Thursday and Friday morning as part of the Met Office's yellow weather warning.
Ferries between Wales and Ireland are preparing for their busiest time of the year with about 12,000 passengers travelling on Irish Ferry services in the build-up to Christmas.
Bosses have warned passengers to travel early to avoid any traffic congestion with the last Irish ferry sailings to Ireland before Christmas at 14:10 from Holyhead to Dublin on Christmas Eve and 14:45 from Pembroke to Rosslare.