Drivers are warned of busy roads and delays at ports in Kent as most schools in England and Wales collapse.
National Highways said the rest of the week and weekend is expected to be one of the busiest times of the year for vacationers.
The ferry operators told passengers to expect delays of several hours at the border.
The RAC has already said it could be the busiest summer holiday since it started collecting figures in 2014.
The Automobile Association estimates UK drivers could make 18.82 million journeys between Friday and Monday, with Saturday expected to be the busiest day.
Data from Inrix suggests that the M25 could experience some of the worst congestion, particularly on routes between Bromley and the Dartford Crossing, Maple Cross to the M3 and the M23 to the M40.
It says the A303 near Stonehenge, the M4 between Cardiff and Newport and the M5 south of Bristol will also see congestion at various points over the weekend.
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Kent has the traffic management system in place known as Operation Brock, with Dover-bound lorries queuing on one side of the M20 while a reverse flow system runs on the remaining carriageway.
National Highways advised people using the Eurotunnel and the Port of Dover to allow extra time for their journeys and to check with their tour operator before departure.
Previously, ferry operator DFDS asked passengers to allow three hours for the check-in process and border controls at Dover.
P&O Ferries also warned of “large queues” of around two hours at border control but later said the queues had been cleared for tourist traffic.
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