Daytrippers and holidaymakers have been warned to expect bank holiday weekend delays, especially around a 60,000-capacity open-air concert in Swansea.
Taylor Swift, Florence and the Machine, Ed Sheeran and Sam Smith are headlining BBC Radio 1's Biggest Weekend at Singleton Park on Saturday and Sunday.
Congestion is also expected around Hay-on-Wye in Powys for the Hay Festival.
Drivers are told journeys will take 30% longer this weekend and trains between south Wales and London will be longer.
Traffic data analysts Inrix have warned journeys to seaside resorts will take longer and one of the main routes to the popular Gower Peninsula runs next to the Biggest Weekend site.
The A4067 Mumbles Road will also close between Sketty Lane and the St Helens sports ground from 21:00 BST to 23:45 on both Saturday and Sunday as concert-goers leave the festival.
Rail passengers are warned to "leave at least an hour" to get to Swansea train station after the event but there will be only one train to Cardiff and Carmarthen after the 22:00 finish.
Traffic experts say road journeys will take a third longer than normal - but Inrix estimate the travel time across north Wales will take almost an hour longer at the A55's weekend peak on Saturday lunchtime.
"Expect the UK's roads to be packed this May bank holiday weekend as many people head for the seaside," said Dr Graham Cookson from Inrix.
"Routes to popular seaside towns will see the most traffic this weekend. Leaving early Saturday morning will see drivers avoiding the worst of the traffic, which peaks on Friday evening."
Rail services between Swansea and London Paddington will take about 40 minutes longer from Saturday to Monday due to upgrade work around Swindon.
The work also means trains on the south Wales main line will not stop at Swindon, Didcot Parkway or Bristol Parkway but will instead call at Patchway in Bristol.
Cardiff Airport said it was expecting more than 22,000 passengers between Friday to Monday with Faro in Portugal, the German city of Munich and Jersey the most popular destinations.
The Biggest Weekend is live across the BBC on TV, radio and online.