Major UK roads were packed from early afternoon on Friday as millions headed off for the August bank holiday.
Airlines said flight schedules would run normally, but tight security continues with travellers limited to one small piece of hand luggage.
Airport operator BAA advised people not to arrive more than three hours before check-in, and to say their goodbyes outside terminals to ease congestion.
Forecasters say the UK will see light showers over the weekend.
The BBC Weather Centre said Monday will bring sunny intervals but there will be no repeat of the heat seen earlier in the summer, with temperatures set to get no higher than 22C.
Earlier major routes were reported to be slow moving due to the weight of the bank holiday traffic, with several accidents adding to delays.
Early hotspots included the northbound M6 in Cheshire and the M5 near Bristol and the M4 westbound near Bath.
Elsewhere, a light aircraft has made an emergency landing beside the M18 at J2 with the A1(M) at Thurcroft in South Yorkshire.
There are reports of fuel spillage on the road and the M18 motorway has been closed in both directions by the emergency services.
Meanwhile, budget carrier Ryanair has announced it is going ahead with its threat to sue the government for £3.3m for losses incurred due to security measures at UK airports.
The anti-terror restrictions mean only one bag no bigger than 45cm x 35cm x 16cm (17.7in x 13.7in x 6.2in approx) may be carried on with the passenger, while everything else must go in the aircraft hold.
No liquids of any type are permitted through the airport security search point, other than prescription medicines and baby milk and liquid baby food.
For flights to the US, extra restrictions are in place and no toiletries or cosmetics purchased in the departure lounge will be allowed on.
The Association of British Travel Agents (Abta) said Friday and Saturday would be the "busiest days" at UK airports, with more than 375,000 people leaving from Heathrow, 250,000 from Gatwick, and 160,000 from both Stansted and Manchester.
A further 70,000 are expected to fly from Luton, 65,000 from Birmingham and 35,000 from Nottingham East Midlands.
AA's TOP 10 BOTTLENECKS
A30 in Devon and Cornwall - Particularly at Bodmin & Carland Cross, near Newquay
M5 Southbound past Bristol - J15-J19; J21, Weston-Super-Mare
M6 around Manchester & Liverpool - J29-J30, Preston; J17, Sandbach
M6 through the West Midlands - J5-J8 Birmingham-Walsall
M5 Northbound in the Midlands - J1-M6, West Bromwich-Walsall
A40 Oxford Northern Bypass - roadworks around Headington
A303 Westbound - Around Stonehenge and at Podimore Roundabout north of Yeovil
M4 Westbound - J1-J5 Chiswick-Slough
M1 Northbound - J6-J10 Hemel Hempstead-Luton [roadworks]; J23-J27 Loughborough-Nottingham
M25 around London - particularly J25-J26 Holmesdale Tunnel, Waltham Abbey [roadworks], J12-J16, Staines to Uxbridge
Around 320,000 will head for the Continent by ferry or by Eurotunnel.
The Eurostar has added an extra two trains to its schedule to cater for the additional 140,000 passengers it is expecting.
Favourite hotspots for British travellers this weekend are Majorca, the Canary Islands, the Greek Islands, Italy, Cyprus and Turkey, Abta said.
On the roads the Highways Agency said it was expecting as many as 30 million vehicles, and is suspending roadworks on 12 sites in England.
Meanwhile the AA has warned of traffic hotspots around the country, including the A30 in Devon and Cornwall, and the M5 southbound at junctions 15 to 19, and 21.
And police in Northern Ireland have warned misbehaving motorists will face a crackdown.
Operation Roadsafe will target motorists who drink and drive, speed, use mobile phones while driving, fail to wear seatbelts or who drive without due care and attention.
BANK HOLIDAY EVENTS
Bath - The Roman Baths by Torchlight
Bedfordshire - The Glenn Miller Festival
Berkshire and W Yorkshire- The Reading and Leeds Festival
Cambridgeshire - St Neots Carnival Week
Derbyshire - Crooked Spire Church Open Day
Devon - Navy Days
Edinburgh - Edinburgh Festival
Hampshire - Navy Days
London - Notting Hill Carnival
Merseyside - International Beatles Week Festival
Early rain should clear on Saturday to be followed by showers and sunny spells, while Sunday will be generally cloudy with occasional drizzle.
Bank Holiday Monday is set to be dry, but cooler with brisk, westerly winds.
Green Flag Motoring Assistance says the majority of breakdowns are caused by poor car maintenance.
The company said it responded to more than 30,000 calls for help last August Bank Holiday weekend.
The firm has advised motorists heading off on long-distance journeys this bank holiday to avoid the busiest time on the roads - between 1000 and 1200 BST.
After the bank holiday, a 24-hour strike on Tuesday 29 August will hit South West Trains on many routes.
SWT advised passengers not to travel on its trains on the day, as the strike "will severely limit services".