Drivers have been advised to "get out early" to avoid the queues during the Easter getaway as almost one million cars are expected on Welsh roads.
Traffic analysts have recommended leaving before 10:00 BST on Good Friday to beat the rush with warnings of hour-long delays at peak times in Wales.
Data suggests a 20% increase in cars on UK roads over the bank holiday weekend.
On the railways, signal work means buses replace trains between Bristol and the south of England all weekend.
So rail journeys to and from south Wales will terminate or start at Bristol Temple Meads.
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Traffic analytics firm Inrix expects more than 950,000 cars on the road in Wales - and 19.5 million cars across the UK - over the Easter period.
The M4 motorway westbound between Caerleon in Newport and the Coryton roundabout north of Cardiff - junction 25 to junction 32 - is expected to be the worst affected stretch of road in Wales with delays of more than an hour and 10 minutes predicted at peak periods.
Delays of more than 30 minutes are also forecast on the M4 westbound between junction 33 for Cardiff West and junction 48 for Pontarddulais, near Swansea, at busy times.
Inrix has also warned that the A477 between St Clears in Carmarthenshire and Pembroke, as well as the A48 and A40 westbound through Carmarthenshire, are expected to be busy at peak times.
The M4 between Magor in Monmouthshire and Bath - junction 23 to junction 18 - is expected to be one of the busiest stretches of road in the UK this Easter.
Meanwhile more than 16,000 passengers are expected to travel through Cardiff Airport over the weekend, about 1,000 people more than last year.
Amsterdam is the most popular destination this Easter with Malaga, Paris and Rome close behind.
Inrix has recommended drivers leave early in the morning or wait until Saturday or Sunday "when roads will actually see 25% less traffic than normal".
"Ranked the third most congested country in Europe, UK drivers are all too familiar with sitting in traffic," said Graham Cookson, Inrix's chief economist.
"This Easter, drivers should expect congestion across a greater number of days than in previous years.
"If drivers have to travel during peak times, they should consider alternative routes and check live traffic data to ensure they avoid the worst of the delays."
Inrix forecasts Thursday and Easter Monday will have a 20% increase in the average number of cars on the road while Good Friday will see a 15% increase.