The AA is warning drivers that the M5 will be one of the busiest roads in the country, ahead of the Christmas weekend.
An amber traffic warning has been issued for both today (23 December) and Christmas Eve.
More than 17 million drivers are expected to embark on journeys to visit friends and family.
Reduced rail services will put more pressure on the roads as train workers prepare to go on strike again.
Congestion is likely to build between 10am and 7pm today, and tomorrow between 10am and 4pm.
The AA has also advised that the M4/M5 interchange at Bristol is likely to be heavily congested.
Jack Cousens, head of roads policy for the AA, said: “As people start driving home for Christmas, we are advising those heading out in their cars to be prepared for some congestion, especially on popular routes.
“The rail strikes have convinced more people to travel by car this year, and while hundreds of miles of roadworks have been removed to ease the pain, it might not be enough to keep the queues away.
“During the recent snow and icy weather, we attended numerous breakdowns which could have been avoided completely had their car been checked before leaving.
“While tyres and oil levels are common items to tick off the list, ensuring screen wash isn’t frozen and wiper blades are in good condition are just as vital, especially with more grit and dirt being kicked up off the road."
Great Western Railway is also warning people not to travel on Christmas Eve unless they ‘really have to’ as workers across the region will walk out over pay and conditions.
Most of the network will close by midday on Saturday 24 December, causing additional build-up on the roads.
It says there is likely to be significant disruption to train services over the entire festive period.