Music fans have been warned to plan ahead and leave extra travel time, with 60,000 people expected to descend on Cardiff for two nights of concerts.
Weeks after three Ed Sheeran gigs at Cardiff's Principality Stadium caused travel chaos, Stereophonics and Sir Tom Jones are there on Friday and Saturday.
On Friday M4 traffic was very slow from J23A (Magor Services) to J26 (Malpas Road), and on the A48 at Newport.
Rail upgrades and routine roadworks are suspended to avoid disruption.
The Welsh government and travel firms have told people to leave in good time.
'We can make a day of it'
The two Stereophonics concerts at Principality Stadium - where Sir Tom Jones is supporting - were rescheduled after the initial dates before Christmas were postponed amid growing concerns about the Covid Omicron variant.
"We're probably the only people who were happy it got cancelled because we can make a day of it," said fan Charlotte Wiltshire, 33, from Porth, Rhondda Cynon Taf.
She said if the concert had gone ahead in December it would have been a "gloomy" after work event.
"As soon as we found it was June we were like, right, let's book the day off."
"It's about time," said Donna Robert, 45, from Tonyrefail, Rhondda Cynon Taf.
"We've always been fans, we've seen them a few times in Reading but this is special, it's home for both of them, so it's great."
'Heaving with people'
Her partner Carl Thomas, 51, called Cardiff a "great venue" on a sunny afternoon.
"It's absolutely heaving with people," he said. "That's why we're sitting outside on the grass and not inside a pub somewhere."
The shows come three weeks after some motorists endured 15-mile tailbacks on the M4 into Wales to watch Ed Sheeran in Cardiff.
People attending the gigs have been advised to leave extra time for the journey to Cardiff.
Meanwhile, the Welsh language music and culture festival Tafwyl at the weekend in Bute Park will swell numbers expected in the city on Saturday.
Roads in Cardiff city centre will be shut for the afternoon before both stadium concerts on "health and safety grounds to ensure people can safely enter and leave the stadium".
Rail passengers have also been told that all services around Cardiff will be busy on both days and that trains will be at full available capacity.
"There's no doubt it will be very busy on Friday and Saturday as it always is when big, sold-out events happen in the Principality Stadium," said Jan Chaudhry-Van der Velde, of Transport for Wales (TfW).
"Every single carriage will be in operation, we've altered our maintenance schedule and have capacity for 20,000 people to get to various locations - 6,000 to the valleys, 6,000 to Bristol and 5,000 to west Wales."
While TfW acknowledged it faced "challenges" on Friday and Saturday, location data from concert ticket sales suggests that about "70% of concert-goers are coming from the "Cardiff, valleys and Swansea area."
"We have plenty of capacity to move people based on estimates provided by the stadium," Colin Lea of TfW told BBC Radio Wales.
🥁 Fans heading into Cardiff on Friday/Saturday for the @stereophonics concerts are being reminded that a queuing system will be in place at Cardiff Central Station.— Trafnidiaeth Cymru Trenau Transport for Wales Rail (@tfwrail) June 16, 2022
Please familiarise yourself with the queuing system 👇
ℹ https://t.co/RdYEhI39pJ pic.twitter.com/F0EFu1C1zk
GWR, the train operator for the south Wales mainline to London, said it would have 31 trains and more than 17,000 seats for customers leaving Cardiff after the two shows.
Railway upgrade work and unnecessary roadworks on the network in south Wales have been suspended for event weekends at the Principality Stadium but travellers have been urged to plan ahead.
"We plan and work to minimise travel disruption on the trunk road network," said the Welsh government.
Travelling to see Stereophonics this friday & saturday?— Traffic Wales South (@TrafficWalesS) June 16, 2022
Daily traffic flows on the #M4 from J28-29 increased by 18% on average during the Ed Sheeran concert. 📈
Traffic peaks on the M4 were between 1pm-7pm👀⬇️
This weekend, plan ahead & leave earlier to avoid delays & stress. pic.twitter.com/dg3mE7biKv
"As with all large events, we would advise people to plan ahead before starting to travel and to allow plenty of time for their journey."
Friday was the hottest day of the year so far in Cardiff - with temperatures reaching 28C (82.4F) - but the Principality Stadium roof was shut.
The Welsh band, whose hits include Maybe Tomorrow, The Bartender And The Thief and their 2005 No 1 Dakota, played the Cardiff city centre venue in 2001 and 2003 but this year they are doing two 60,000-capacity shows after an extra date was added due to demand.
'Something to look forward to'
"A few of the boys I went to school with said "it'll sell out within half an hour" but I said it'd take weeks because it's a hard venue to sell out," said Kelly.
"Then at half past nine on the day the first date went on sale, I had a phone call to say we've sold out. So the demand was so big, the advice was we could definitely do good numbers on both nights.
"It's been an idea to play the Millennium Stadium for a couple of years so do it with such amazing special guests and after a global pandemic it couldn't have come at a better time for people to have something to look forward to."
Ashton Adams has seen the band 12 times and after watching Wales qualify for football's World Cup, he is going with a bunch of friends.
"I think this may be the best two weeks to be Welsh in history," said Ashton, who grew up near where the Stereophonics are from in Cwmaman, Rhondda Cynon Taf.
"What's brilliant is the absolute timing of this gig, we've got the traditional Welsh music festival in the castle, whilst Welsh legends play the stadium, on the back of the Welsh football, it's just the best time and it's incredible.
"It's been long awaited and we've all been through a hard time with Covid so it'll be great to have live music back. It makes you feel like the world's getting back to normal."
What time does the concert start?
Feeder, whose hits include Buck Rogers, Just A Day and Just the Way I'm Feeling, will also be on Friday's bill while Super Furry Animals frontman Gruff Rhys will also support on Saturday - with Sir Tom Jones starting his set at 18:15 BST while the Stereophonics will be on at 20:00.
Gates to the Principality Stadium will open at 15:00 and the first act is Welsh group Buzzard Buzzard Buzzard who will start the show on both days at 15:40.
Take That are currently the only group to have played three headline shows in different years at the Principality Stadium but Stereophonics, whose 12th studio album earlier this year was their eighth number one record, will emulate that this weekend.