Britain's Christmas getaway began smoothly on the roads, with few delays reported in what was expected to be the busiest time of the festive period.
Millions of cars will take to the roads over the festive season
Some 18m cars were expected on the roads during Friday, with 16m likely on 27 and 28 December.
But rail travellers faced severe problems because the West Coast Mainline is closing for five days.
And in the evening fog disrupted flights from Gatwick and Southampton airports, mostly to Europe.
A total of 16 departures were cancelled at Gatwick airport, with as many as 2,000 people affected.
All flights leaving Southampton after 1700 GMT were stopped, with arrival diverted to other airports.
Incoming flights also had to be diverted away from both Stansted and London City.
Meanwhile, UK airport workers have voted for industrial action, which will be taking place on 7, 14 and 17 January.
The dispute with BAA, which operates seven UK airports including Heathrow and Gatwick, is over the closure of final salary pensions to new employees.
A record 3.5m Britons will head abroad for Christmas.
Christmas Day should be much quieter on UK roads, with only 4m cars being driven.
The Highways Agency is suspending roadworks on several motorways and major A-roads in England over the holiday period, but roadworks remain on some key routes, including the M1 and M25.
A spokesman said there had been delays on some roads.
Drivers on the M1, southbound between junctions four and two, saw 50-minute tailbacks.
There were 40-minute delays on the M40, northbound between junctions nine and 10, and the A34 was closed northbound near the M4, causing 40-minute delays.
Adam Cracknell from the RAC said there would be a high volume of traffic on the roads on the 27 and 28 December because of the post-Christmas sales.
"On those days people will be going back to work but they'll also be going sale shopping, so before noon there will be a lot of congestion on the nation's roads, but that shouldn't be as bad in the afternoon and evening.
"People tend to spend more time with the family on Boxing Day, but after that the bargain mist comes down."
Kevin Groves, of Network Rail, said rail travellers on the West Coast Mainline would experience disruption because the company had "made a mistake" and under-estimated the amount of work being planned over the Christmas break.
As a result, engineering work beginning on 27 December will now last until New Year's Eve, a day later than originally planned.
The work will mean the closure of the West Coast Mainline, which affects any services that would have passed through Rugby, disrupting travel between London and Scotland, Manchester and Birmingham.
These are mostly Virgin Train's services, but the closure also affects some London Midland and First ScotRail Sleeper services.
Alternative services will be provided either by other train operating companies or replacement coaches.
The Association of Train Operating Companies (Atoc) said up to 4m passengers, about one million more than a typical weekday, were expected to take the train on Friday, and Saturday is also predicted to be busy.
George Muir, its director general, said: "Christmas Day falls this year on a Tuesday and with more and more people taking two weeks off, we are expecting many passengers to travel on the Friday and Saturday before Christmas.
A spokeswoman for the Met Office said conditions on the roads would be "tricky" on Saturday morning.
"We can expect a frost on Friday night in southern England and quite a sharp frost over south-east England and East Anglia by Saturday morning.
"Northern England and Scotland has the biggest potential for travel problems. There is a risk of ice, mostly over Scotland, with snow over mountains in Grampian.
"That risk continues through the higher ground in northern England, places such as Lancashire, North Yorkshire, Cumbria, Northumberland and Durham.
"Those icy conditions will last during Saturday morning and could cause problems for Scottish airports in Glasgow, Aberdeen and Edinburgh."
Steve Crosthwaite, head of the National Traffic Control Centre, said patchy fog was possible in many areas on Saturday morning, as well as freezing rain in the West from Birmingham to the Scottish Borders.
Temperatures are expected to rise on Sunday, with only a light frost in England and Wales and rain and wind in Scotland.
For those heading abroad, top city-break destinations include Paris, Amsterdam, Dublin and New York, according to the Association of British Travel Agents (Abta).
The Canary Islands, Egypt, Goa and Dubai are also proving popular among travellers this year. Abta also said the busiest day for overseas travel will be Saturday, followed by 30 December when people head for New Year's Eve celebrations abroad.
A total of 1.2m passengers are expected to leave from London's Heathrow airport between Friday and 2 January.