British drivers will make an estimated 20 million car journeys to see friends and family in the run-up to Christmas Day.
The RAC said traffic will increase steadily from Monday before peaking on Friday and Saturday, Christmas Eve.
Pressure on the streets will be increased by a strike by thousands of Rail, Maritime and Transport Union (RMT) members at Network Rail starting at 6pm on Christmas Eve.
Rail travelers are encouraged to finish their journey as early as midday that day, meaning many people will choose to travel by car or bus instead.
How does traffic affect the roads?
The RAC forecast that 7.9 million escape trips will be made in the two days immediately leading up to Christmas Day.
The worst traffic jams on these days are expected between 10 a.m. and 7 p.m. on Friday and between 12 p.m. and 1 p.m. on Saturday.
Total traffic levels will be far greater ahead of the weekend as holiday drivers compete for road space with commuters and everyday drivers – likely causing congestion in cities and on major thoroughfares.
The traffic analysis company Inrix expects journey times to be around 14% longer than in the same period last year.
Roads likely to face congestion this week include the M25, the M60 near Manchester, the M6 in north-west England and the M40 in Oxfordshire.
National Highways said it will ensure nearly 98% of England’s motorways and major A roads are fully open from 6am Tuesday until the end of January 2 by either completing or suspending roadworks.
When is the best time to travel?
RAC spokesman Rod Dennis said: “Our research suggests that escape traffic will increase steadily throughout the week, culminating in two hectic days of travel just before Christmas itself.
“By Friday, we fear there will be congestion on the roads today and tomorrow as millions jump behind the wheel to see family and friends – with last-minute Christmas shoppers as well as the fallout from another rail strike resulting in a increase in traffic.”
The RAC urged drivers to take their rides on quieter days if possible this week.
Read more on Sky News:
Strikes every day before Christmas – which sectors are affected
Dates for Christmas and January Train disruption and all affected lines
The RAC’s traffic estimates are based on a survey of 1,886 UK adults conducted last week.
The AA, which issued a “yellow traffic warning” for Friday and Saturday, urged motorists to run car checks including fuel, oil, coolant and windshield washer fluid, and to make sure tires are properly inflated.
The company’s head of road transport policy, Jack Cousens, said: “Breakdowns contribute to congestion. So if everyone would spend 10 minutes checking their car before leaving, it could save everyone hours on their trip.”