More than 12 million cars will be taking to the roads over the December 24-26 period, the RAC has predicted.
Before then, the big getaway is expected to start in earnest on Monday, with drivers taking advantage of some of the lowest petrol prices for four years.
With unsettled weather likely, the RAC urged drivers to prepare well for journeys.
The AA and the Environment Agency reminded motorists to stay out of flood water as they head off, after revealing that last Christmas Eve was the busiest day in the last five years for flood rescues.
The RAC said around four million cars will be on the road on Christmas Eve and about 4.6 million on Christmas Day.
RAC national operations manager Matt Dallaway said: "December has seen the UK's weather take on a distinctly unsettled and for some of us wintry flavour, and the signs are that this could continue through the week when many of us will be taking to the roads.
"So the message is for drivers to plan for their journeys ahead of time, and make sure their vehicles are properly winter-proof."
The AA said it rescued 642 vehicles from flood water on Christmas Eve last year, while the Environment Agency issued 450 flood warnings and alerts on the same day.
The AA said it only takes water 11.8in deep to float a car yet more than a third of nearly 20,000 AA members polled said they would have no qualms about driving through this level of water.
As many as 8% would be happy to drive through water as deep as one metre (more than 39 inches).
John Seymour, national manager of the AA's flood rescue team, said: "In the Christmas dash, it's understandable that sometimes people's minds are elsewhere but no one wants to be writing a last-minute letter to Father Christmas after wrecking their car in flood water."
As many as four million Britons are heading overseas over the festive period, slightly more than last year. T op winter sun destinations for Britons included the Canary Islands, Tunisia and Morocco, while leading long-haul destinations included Dubai, Mexico, Cuba and Egypt.
Popular for New Year's Eve breaks are the cities of Paris, Amsterdam, Barcelona, Berlin and Dublin, while domestically Edinburgh and London lead the way.
Popular destinations include the Portuguese Algarve, the Canary Islands, Cologne and Prague.
Those staying at home will have to contend with disruption caused by a £200 million rail engineering programme over the holiday period.
Among routes being disrupted is the West Coast line where major work is going on at Watford and between Stafford and Crewe.
There is also work at Reading, while tomorrow will see the start of a 16-day part-closure of London Bridge station for work involving the £6.5 billion Thameslink programme.
Network Rail and industry body the Rail Delivery Group have stressed the need for the work to go on during a time when few commuters travel.
But customer watchdog Passenger Focus said: "F or passengers wanting to travel over the Christmas period, engineering works can be a major inconvenience, especially if it means having to use rail replacement bus services."
The Highways Agency will be suspending, or will have completed, almost 400 miles (644km) of roadworks in time for the final part of the Christmas getaway.
The agency aims to complete or lift 156 schemes by 6am on December 23, although a number of works on busy roads will have to remain in place for safety reasons.
All told, a total of 219 schemes will be in place over the holiday period compared with 172 last year.
The agency will complete 264 miles (425km) of roadworks at 107 locations before the holiday. A further 49 sets of roadworks and lane restrictions will be suspended across 118 miles (190km) of carriageway from 6am on December 23 until midnight on Friday January 2.
Among locations where roadworks remain in place are various sections of the M1, including works at Luton in Bedfordshire and Rugby in the West Midlands.
Other sites include the M4 near Reading in Berkshire, the M5 near Gloucester and the M6 near Birmingham.