UK drivers will embark on an estimated 20 million car trips to see friends and family in the run-up to Christmas Day.
The RAC said traffic will build steadily from Monday before peaking on Friday and Saturday, which is Christmas Eve.
Pressure on the roads will be heightened due to a strike by thousands of members of the Rail, Maritime and Transport union (RMT) at Network Rail from 6pm on Christmas Eve.
Train passengers are being urged to complete journeys as early as lunchtime on that day, meaning many people will opt for car or coach travel instead.
How will the traffic impact roads?
The RAC predicted that 7.9 million getaway trips would be made over the two days immediately before Christmas Day.
The worst congestion on those days is expected to be between 10am and 7pm on Friday, and between noon and 1pm on Saturday.
Total traffic volumes will be far greater prior to the weekend as drivers getting away for the festive period compete for road space with commuters and everyday motorists – this is likely to cause jams in cities and on major routes.
Transport analytics company Inrix expects journey times to be around 14% longer compared with the same period last year.
Roads likely to be hit by congestion this week include the M25, the M60 near Manchester, the M6 in northwest England and the M40 in Oxfordshire.
National Highways said it will ensure almost 98% of England’s motorways and major A-roads are fully open from 6am on Tuesday until the end of January 2 by either completing or lifting roadworks.
When will it be the best time to travel?
RAC spokesman Rod Dennis said: “Our research suggests getaway traffic will build steadily all week, culminating in two frantic days of traveling just before Christmas itself.
By Friday, we fear it will be a case of jams today and jams tomorrow on the roads as millions of people jump behind the wheel to see family and friends – with last-minute Christmas shoppers as well as the impact of another rail strike also. leading to an increase in traffic.”
The RAC urged drivers to take their trips on quieter days this week if possible.
Read more on Sky News:
Strikes every day before Christmas – which sectors are affected
Dates for Christmas and January train disruption and all the lines affected
The RAC’s traffic estimates are based on a survey of 1,886 UK adults conducted last week.
The AA, which issued an “amber traffic warning” for Friday and Saturday, urged motorists to carry out car checks such as levels of fuel, oil, coolant and screen wash, and ensure tires are correctly inflated.
The company’s head of roads policy, Jack Cousens, said: “Breakdowns add to traffic jams, so if everyone spent 10 minutes checking their car before leaving it could save everyone hours on their journey.”