Drivers are being warned to expect huge queues this week as the busiest summer getaway in five years begins.
Researchers predict there will be 13.4 million holiday journeys between Friday and Sunday, four million more than last year.
Friday, when many schools break up, is expected to be the worst day for congestion.
Severe hold-ups are tipped on major routes.
To avoid making congestion worse, the RAC’s Ben Aldous urges drivers to check oil, coolant and tyres before setting off.
He said: “The last thing any family wants is a breakdown spoiling the start of their holiday.
“Jams are pretty much guaranteed and every summer we see extra delays caused by broken down vehicles.”
Highways England’s head of road safety, Richard Leonard, added: “Breakdowns are still too common.
"By doing checks before setting off, motorists can make sure they keep everyone moving.”
The RAC and traffic experts Inrix say this weekend will be the biggest summer getaway since 2014.
The M1 southbound from J16 to J6 is likely to see the heaviest traffic, with 90-minute delays.
Hold-ups of around an hour are predicted for the M25 anticlockwise from J4 to J1.
Long queues are also forecast on the M40, M5 and M6.
Drivers who want to avoid the worst of Friday’s traffic are advised to travel after 8pm.
