Millions of families embarking on summer getaways are being warned to expect delays over the coming days.
Traffic jams are likely on major roads, while the Port of Dover is warning it could take up to two-and-a-half hours to pass through border checks.
The majority of schools in England and Wales break up for summer on Friday, sparking a surge in holidays.
The RAC estimated that 12.6 million car journeys will be made for a day trip or holiday between Friday and Monday, causing “bumper-to-bumper traffic”.
Transport analysis company Inrix predicted that traffic hotspots will include the M5 south from near Bristol (junction 15) to Bridgwater (junction 23), which is a popular route for holidaymakers travelling to the South West.
Another motorway stretch likely to experience hold-ups is the M25 clockwise between junction 10 for the A3 to Kingston and junction 6 for the A22 to East Grinstead.
And the Met Office has warned there could be travel disruption on Saturday as persistent rain is forecast followed by a risk of thunderstorms on Sunday.
Drivers wanting to avoid long queues over the coming days are advised not to travel on major roads between late morning and early evening.
The AA advised motorists to carry out checks such as oil and coolant levels, and tyre pressure, to reduce the likelihood of their car breaking down while being used for a summer holiday.
Nick Powell, AA patrol of the year, said: “The low mileages that city cars tend to do, particularly in London, make it more difficult for their owners to pick up on things that may be going wrong with their vehicles.
“Go on a journey of hundreds of miles, with the summer holiday challenges of heat and traffic jams, and hidden faults such as tyre and coolant problems can easily spring up and ruin a holiday.”
On Thursday “severe” traffic delays were reported by National Highways in more than a dozen areas due to congestion.
Some of the roads included the M42 northbound between junctions 3A south of Birmingham and junction five for Solihull, the M23 westbound between junctions nine and 9A near Gatwick and the M6 northbound between junctions 19 at Tabley, near Knutsford in Cheshire and 20 at Lymm in Cheshire.
The Port of Dover is expecting more than 6,000 cars to arrive on Friday, followed by a further 7,500 on Saturday.
It said border control processing will generally take around one and a half hours on peak days, but warned this could rise to two and a half hours between 6am and 1pm during the next two weekends due to the number of passengers.
Passengers travelling between Thursday and Sunday are being asked to arrive up to three hours before sailings.
Travellers are advised to carry plenty of food and drinks.
Enhanced post-Brexit passport checks by French border officials Police Aux Frontieres have significantly increased processing times at the Kent port.
On Thursday morning the average border processing time for tourists was 50 minutes, the Port of Dover tweeted, but it had reduced to 10 minutes by 10am.
By 4.40pm the port said that 17,783 people had crossed the border to France and there was no wait time ahead of border controls.
