Motorists are being warned to expect severe congestion on major routes this week as the busiest summer getaway in five years begins.
Research by the RAC and traffic information supplier Inrix indicates that 13.4 million leisure trips will take place by car between Friday and Sunday.
This is around four million more than the same weekend in 2018 and would represent the largest summer getaway since 2014, according to the analysis.
The worst day of the week for congestion is expected on "frantic Friday" when many school terms end.
Some 5.3 million drivers embarking on day trips or longer breaks will battle for road space with regular commuters.
Researchers say the combination of usual commuter traffic and the beginning of the school holidays means delays of up to an hour and a half on major roads.
Traffic will slow to an average of 7mph on some parts of Britain’s road network, the data warned.
The M1 southbound from Junction 16 at Northampton to Junction 6 at north Watford is likely to see the longest jams, with delays of 89 minutes.
Hold-ups of around an hour are predicted for the M25 anticlockwise from Junction 4 at Bromley to Junction 1 at Swanscombe/Dartford.
Significant queues are also forecast on the M40, M5 and M6.
Drivers are advised to travel after 8pm if possible to avoid the worst of the traffic.
RAC patrol of the year Ben Aldous urged drivers to carry out basic car checks such as oil, coolant and tyre tread and pressure before embarking on a long trip.
He said: "The last thing any family wants is a breakdown spoiling the start of their summer holiday they've no doubt been looking forward to for a long time.
"Traffic jams are pretty much guaranteed and while it's possible to predict where some of these will be, every summer we see extra delays caused by broken down vehicles blocking lanes, leaving drivers faced with hours of frustration."
Highways England's head of road safety Richard Leonard added: "Breakdowns are still too common. We'd urge motorists to check their vehicles on a regular basis and this week is really important because we know that many schools break up for summer and people are setting off on holidays and getaways.
"By doing the correct checks before setting off, motorists can make sure they keep everyone moving which means everyone can reach their destination safely and on time."