The Christmas getaway is being hit by congested roads and disruption on the rail network.
Highways England reported 46 traffic incidents on motorways and major A-roads at around 1pm, including 15 classed as severe.
An estimated 1.3 million drivers on leisure trips are fighting for road space with regular commuters on the last working day before Christmas.
Graham Cookson, chief economist at transportation analyst Inrix, warned it will be one of the busiest days of the year with “incredibly severe” traffic jams.
He told the Press Association all major motorways will see congestion and “it’s tricky to avoid”.
He said the M25 is “right at the top of the list”, warning: “It doesn’t matter which way you’re going round it.”
The M6 between Merseyside and Staffordshire, the M1 and the M4 were also expected to be “incredibly busy”.
The M6 between Merseyside and Staffordshire is expected to be among the busiest roads (Joe Giddens/PA)
The RAC dubbed the day Frantic Friday and strongly urged drivers to avoid long journeys “if they possibly can”.
It estimated that 11.5 million leisure trips will be made by car between December 17 and Christmas Eve, but this will be eclipsed by the 17.5 million between Christmas Day and New Year’s Day as people take advantage of post-Christmas sales at shops.
Almost 400 miles of roadworks have been lifted to ease journeys on motorways and major A-roads in England until January 2.
On the rail network a number of operators reported delays and cancellations.
South Western Railway said around 30 services could be cancelled and run short due to “sickness among train crew”.
A spokesman said: “We know how important today is for passengers travelling home for the holidays and we will cover as many of these as possible.”
ScotRail suffered delays and cancellations on the route between Stranraer/Ardrossan Harbour and Kilmarnock/Glasgow Central after a train derailment in a depot meant fewer trains could run.
A broken-down train caused disruption to Northern trains between Workington and Whitehaven.
A strike by rail workers on Virgin Trains was called off on Thursday, but a 24-hour walkout went ahead by members of the Rail, Maritime and Transport union on Merseyrail, leading to a reduced service across its network on Friday.
Many of Britain’s mainline routes will be partially shut in the coming days as Network Rail carries out its biggest ever Christmas engineering programme.
Great Western Railway is urging passengers to complete journeys by Saturday “at the latest” as London Paddington will be closed between Christmas Eve and December 27.
(PA Graphics)
The West Coast main line will be halted between Preston and Lancaster from Christmas Eve to December 27, with buses replacing trains.
Passengers travelling between London and Glasgow during this time are advised to go via Edinburgh, adding around an hour to journeys.
Coach operator National Express is running its largest ever Christmas schedule to meet increased demand from key locations such as Bournemouth, Cardiff, Leeds, Edinburgh, Portsmouth, Bristol, Glasgow and Manchester.
Airports, ports and international train stations are exceptionally busy with more than 4.5 million people heading abroad over Christmas and the new year.
The UK’s busiest airport, Heathrow, will welcome almost 130,000 departing passengers on Friday.