Drivers warned of delays around Dover as summer getaway begins
Drivers are being warned of busy roads and delays at ports in Kent as most schools in England and Wales break up.
National Highways said the rest of the week and the weekend were expected to be one of the busiest periods of the year for holidaymakers.
Ferry operators told passengers to expect delays of several hours at the border.
The RAC has already said it could be the busiest summer getaway since it started tracking figures in 2014.
The motoring organisation estimates UK drivers could make 18.82 million trips between Friday and Monday, with Saturday expected to be the busiest day.
Data from Inrix suggests the M25 could see some of the worst traffic jams, particularly stretches between Bromley and the Dartford Crossing, Maple Cross to the M3, and the M23 to the M40.
It says the A303 near Stonehenge, M4 between Cardiff and Newport and M5 south of Bristol are also likely to see queuing traffic at various points during the weekend.
In Kent, the traffic management system known as Operation Brock is in place, with lorries heading to Dover queuing on one side of the M20, while a contraflow system runs on the remaining carriageway.
National Highways advised people using Eurotunnel and the Port of Dover to allow more time for their journeys and check with their travel operator before setting out.
Earlier, ferry operator DFDS urged passengers to allow three hours to complete the check-in process and border controls at Dover.
P&O Ferries also warned of "major queues" of around two hours at border control but later said queues had cleared for tourist traffic.
Jack Cousens, head of roads policy for the AA, said people should be prepared for lengthy delays and urged them to check their car before setting off to avoid unnecessary breakdowns and to make sure they have enough fuel.
Drivers should also take plenty of water and snacks for the journey," he added.