Millions travel for bank holiday weekend
About 5.1 million people will be heading off on breaks within the UK over the bank holiday weekend, tourism agency Visit England has estimated.
The AA says traffic is likely to begin building up on Friday evening but the busiest single day for motorists is expected to be Saturday.
Visit England said about £1.3bn is set to be generated for the economy.
Meanwhile, travel organisation Abta expects two million Britons will head abroad between Friday and Monday.
Heathrow is expecting more than 440,000 passengers to depart over the long weekend, while Gatwick is preparing for 311,000 outgoing passengers.
Stansted is due to see 103,000 people fly over the next three days.
Ports and the Channel Tunnel are also expected to be in high demand.
Public nuisance charge
The AA estimates 13 million drivers will take to the roads over the holiday weekend, with about 10 million beginning journeys on Saturday.
Potential hotspots are said to include sections of the M25, M6, M4 and M27.
On Thursday, motorists were caught in long tailbacks on the M5 after an incident on a bridge led police to close part of the motorway near Wellington in Somerset.
Abigail Wyatt, 25, of Tonedale, Somerset, has been charged with causing a public nuisance and will appear before magistrates in Taunton next month.
The M5 was already expected to be exceptionally busy as holidaymakers began to head to the South West for the August bank holiday weekend.
The AA's Max Holdstock said the August Bank Holiday weekend is "always a bit of a mad scramble on the roads".
He added: "Traffic is likely to build up on Friday evening as people set off early to make the most of the weekend, and will be busy again for the return on Monday afternoon - so allow plenty of extra time if you're travelling then."
Highways England says almost 98% of England's motorway and major A roads will be clear of roadworks over the weekend, after some 373 miles of works were either completed or suspended ahead of the holiday.
Hundreds of thousands of people will travel by coach, with operator National Express announcing it will be its busiest weekend of the year.
But nearly 1,000 engineering projects are being carried out across Britain's rail network, meaning some lines will be closed.
Network Rail's route managing director, Martin Frobisher, said: "Work takes place 365 days a year as part of our Railway Upgrade Plan but we carry out larger upgrades over bank holidays when there are fewer passengers travelling."
Abta chief executive Mark Tanzer said: "This weekend is the traditional curtain closer for the peak summer months and it is always a very busy weekend for travel, with millions taking advantage of the long weekend to head off overseas.
"With the roads predicted to be extremely busy, holidaymakers should make sure that they leave plenty of time to get to their port of departure."
Monday is a day off in England, Wales and Northern Ireland, but a working day in Scotland, which had its summer bank holiday at the beginning of the month.