Drivers were warned today to expect traffic build-ups on major routes as the school summer holidays begin this weekend.
The AA said jams were likely from Friday lunchtime, with around 14 million cars expected on the roads.
And road safety minister Mike Penning urged drivers to check traffic conditions before setting off.
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.
Mr 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.”
British Airways released its top 10 holiday getaways for this summer, based on July/August 2011 bookings for holiday packages)
1. New York
2. Orlando, Florida
7. Las Vegas
8. St Lucia
Meanwhile, a survey by Confused.com showed motorists reckoned slow drivers were the biggest frustration for road users.
The poll, of 2,000 motorists, also showed nearly half of UK drivers attempt unsafe overtaking to pass slow-moving vehicles.
As many as 37% said minimum speed limits should be imposed on all roads, while 15% wanted slow drivers to only be allowed to drive at certain times.