The RAC said trips to the seaside would account for lots of the traffic
Busy roads, rail disruption and wet and cold weather for some are predicted for the start of the Bank Holiday weekend.
Motoring organisations estimate up to 18 million cars will take to the roads, with a number of sporting events adding to the volume of holiday traffic.
More than 20% of the rail network is closing for engineering work, including parts of the West Coast Main Line.
Weather forecasters say the north of the UK is expected to stay dry, with heavy rain and cold for the south.
Saturday will be the best day for the UK as a whole, said BBC forecaster Chris Fawkes, with rain and wind sweeping into much of England and Wales over Sunday and Monday.
Engineering works are scheduled at 30 locations on the railways over the weekend.
There will be no Virgin Trains services in and out of London's Euston station on Saturday, Sunday or Monday, and no London Midland trains out of the station on Sunday and Monday due to the long-running upgrade of the West Coast Main Line.
Network Rail said more than 4,000 people would be working on improvements over the three days.
Meanwhile the RAC predicted 18m cars would take to the roads and suggested drivers allow 50% more time for journeys.
And those drivers will pay on average £14 more to fill up their tanks, compared with this time last year, according to the AA.
The RAC said routes expected to be particularly busy were those to the West Country, to the Lake District and to Blackpool, plus routes from northern England to north Wales, and between the Midlands and Norfolk.
Routes leading to sporting venues could also get congested, said the RAC.
Among the events this weekend are the Football League play-offs at Wembley Stadium, the Scottish FA Cup Final in Glasgow on Saturday, the England v New Zealand Test Match in Manchester and the Rugby Union Heineken Cup Final in Cardiff on Saturday.
The RAC's Iain Vale said this was also the weekend that would see traffic to holiday resorts pick up with the start of school's half term.
But on Friday evening, there were no major motorway closures and traffic was flowing freely after the busiest period of 1600 BST, traffic monitoring service Trafficmaster said.
Temperatures could reach 21C (70F) in some parts over the holiday, said Chris Fawkes.
He said Saturday would be mainly dry with sunny spells and breezy for everyone, save for a few showers in the south and west.
On Sunday, rain will spread across southern England, then to Wales, the Midlands and East Anglia before reaching northern England in the afternoon.
"It's going to be a typical Bank Holiday," he said, adding that there was a chance of thunder.
Scotland, Northern Ireland and parts of northern England will stay dry and bright.
Monday spells more of the same, with heavy persistent rain and temperatures in the low teens in the south, and much better weather in the north.
"It's going to feel miserable outside in England and Wales - Scotland and Northern Ireland are the places to be," added Mr Fawkes.
On the railways, maintenance work other than the West Coast Main Line includes platform extensions at Luton Airport Parkway, track and signalling works at Merthyr and Newport, south Wales and infrastructure improvement work at Hull Docks.
NR director of infrastructure investment Simon Kirby said despite disruption the majority of services would continue to run over the holiday weekend.