The travel getaway is due to start today as school summer holidays begin.
But the weather is still not playing ball, with more showery and unsettled conditions predicted over the next few days.
Drivers were warned to expect traffic build-ups on major routes from this lunchtime, with around 14 million cars expected on the roads.
The AA said routes to south west England were likely to be the busiest, with congestion also expected on main routes to the coast, at cross-Channel ports and at major airports.
The top five traffic hotspots were likely to be:
:: The A303 from the M3 through Hampshire, Wiltshire and Somerset;
:: The M5 from Almondsbury near Bristol to Exeter;
:: The M3/M27/A31 from Winchester in Hampshire to the Dorset coast;
:: The M6 between Junction 11A through Staffordshire and Cheshire into Lancashire;
:: The western part of the M25 between M1 interchange at junction 21 in Hertfordshire and the M3 interchange at junction 12 west of Chertsey in Surrey.
AA patrolman of the year Keith Miller said: "Allow extra time for your journey and, if you can, avoid travelling at peak times. Carry plenty of drinking water, food and things to keep the children entertained."
The Highways Agency urged families to plan their journey and to check traffic and weather before setting off.
Road safety minister Mike Penning said: "For many people, this will be the longest car journey of the year, often on an unfamiliar route, and we want to help them to avoid spoiling the start of their holiday.
"A few moments checking traffic conditions before you set off could take a lot of stress out of the journey. There are also plenty of ways to check for updates whenever you stop for a break, which you should do every two hours."
Victoria Kettley, a forecaster with MeteoGroup, the weather division of the Press Association, said: "There are showers across Wales and the west at the moment, and these are going to move east and south east during the day. There may be some sunny spells, but it will be showery.
"Saturday is looking drier for many, but a low pressure system moving down from southern Scandinavia through the North Sea is expected to bring strong winds and rain to eastern parts, for example East Anglia, on Saturday afternoon, and there will be more rain in the east on Sunday.
"Temperatures will remain unspectacular - we are looking at the high teens."