Drivers face "carmageddon" this weekend with the worst bank holiday congestion on UK roads for years, experts warned.
Transport information supplier Inrix said some stretches of road could have twice as many vehicles between Friday and Monday compared with a normal weekend.
Hotspots to avoid including sections of the M25, M5, M27 and M6.
The combination of rail engineering work and a forecast of potentially warm weather is expected to boost the number of journeys over the popular getaway weekend.
Inrix traffic analyst Greg Hallsworth said: "This bank holiday could see the worst congestion in recent years.
"A perfect storm of conditions will lead to increased traffic as carmageddon blights the long weekend.
"With some uncharacteristically good weather forecast and potential rail disruption, holidaymakers could find roads gridlocked far more than usual.
"We have already seen delays of up to 90 minutes during the bank holidays earlier this year but we could see delays of more than 120 minutes for some of the worst affected roads this weekend."
The RAC estimates that 20.7 million cars will be using major roads for leisure trips between Thursday and Monday.
Based on a survey of 1,579 motorists, it expects Saturday is set to be the busiest day with five million leisure journeys planned - double the figure for the same day last year.
Sunday is also set to be busy on the roads with 4.4 million getaway trips.
Road congestion could be boosted with nearly 1,000 engineering projects being carried out across Britain's rail network, meaning some lines will be closed.
Major work is taking place to upgrade signalling in the Bristol area, affecting Great Western Railway passengers.
Other work includes track replacement disrupting services between Milton Keynes and Rugby, and rail replacement buses operating between Preston and Bolton due to a project to electrify the railway through Chorley.
Martin Frobisher, route managing director for Network Rail, said: "Work takes place 365 days a year as part of our Railway Upgrade Plan but we carry out larger upgrades over bank holidays when there are fewer passengers travelling."
Forecasters at the Met Office said temperatures could reach 30C in some places this week, with inland parts of south-east England and East Anglia expected to enjoy the warmest weather.
:: These are the top 11 congestion hotspots to avoid over the bank holiday weekend (expected delay in brackets), according to Inrix:
1. M25 between J9 Leatherhead and J21 Winch Hill Wood (90 minutes)
2. M5 southbound from J14 Thornbury and J22 Highbridge (90 minutes)
3. M25 between J4 Orpington and Dartford Tunnel (50 minutes)
4. M27/A31 between Southampton and Ringwood (50 minutes)
5. M4 westbound from London to the West Country (50 minutes)
6. M6 between J19 Knutsford and J22 Warrington (45 minutes)
7. M6 northbound from Keele to Knutsford (40 minutes)
8. A303 westbound from Andover to Amesbury (40 minutes)
9. M5 between J4a Bromsgrove and J7 Worcester (30 minutes)
10. M25 between J29 Codham Hall Wood and the QEII Bridge (30 minutes)
11. M60/M62 between J8 Carrington and J20 Rochdale (20 minutes)
Highways England announced that almost 98% of England's motorway and major A roads will be clear of roadworks over the weekend.
Some 373 miles of roadworks will either be completed or suspended ahead of the holiday.
The decision was welcomed by the Freight Transport Association, which said lifting "such a significant number of roadworks" was good news for all motorists.