Drivers are being warned to avoid embarking on a bank holiday getaway on Friday afternoon.
Transport analytics firm Inrix said motorists should set off in the morning or after 7pm to miss major traffic jams.
The RAC estimated that 16.7 million leisure trips on UK roads are planned between Friday and Monday.
It added that hopes of good late summer weather could see even more people jumping in their car for a day trip or last-minute break.
Inrix said there could be a 25% increase in delays on Friday compared with a typical August day as millions of people embark on a bank holiday getaway.
The South West is expected to bear the brunt of the traffic over the weekend, with journey times on classic holiday routes such as the M5 and A303 more than doubling at certain times.
RAC Breakdown spokesman Rod Dennis said: “It’s been an incredibly busy summer on the UK’s roads with vast numbers of people on the move both for staycation holidays and for day trips to the sea and countryside.
“Our figures suggest this is a trend that will continue, with bumper-to-bumper bank holiday traffic expected, especially on the most popular holiday routes.
“While this weekend will see many people returning from summer holidays in the UK, the fine and settled weather will also be seen as an opportunity by lots of people to jump in their cars for last-minute trips – after all, this is the last bank holiday weekend in England, Wales and Northern Ireland before Christmas.”
Mr Dennis urged motorists to carry out essential checks on their car, such as ensuring tyres are properly inflated and have plenty of tread, and that oil, coolant and screenwash are at the correct levels.
Inrix analyst Bob Pishue said: “Drivers hitting the road for one last getaway before the end of summer should expect long delays on key corridors.
“Knowing when and where congestion will build can help drivers avoid the stress of sitting in traffic.
“Our advice is to avoid travelling in the afternoon. Leave early in the day or be prepared for some travel times to double.”
AA president Edmund King said: “Weather is always a big decider on where people spend their bank holiday. However, AA research suggests that local traffic may be heavier this time.
“Covid’s disruption to people’s lives is still noticeable in the 10% fewer long-distance trippers this August.
“The drop in longer trips might be due to the difficulty in booking accommodation away from home due to more families taking their holidays in the UK.”
National Highways, recently rebranded from Highways England, said around 98% of England’s motorways and major A roads will be free of roadworks over the weekend.