Traffic management system Operation Brock has been deployed in Kent ahead of the Easter getaway period.
National Highways said the contraflow was in place at 6am on Thursday.
The scheme is designed to manage traffic flows on the M20 motorway in the event of delays at Channel ports, which will see high demand from holidaymakers in the coming days as most schools in England are breaking up for Easter.
It has been introduced on a 13-mile stretch of the motorway between Junction 8 for Maidstone and Junction 9 for Ashford.
Lorries heading for the Port of Dover or Eurotunnel are being directed to follow signs to join a specific lane at Junction 8.
All other traffic for the coast crosses over to enter the contraflow on the London-bound carriageway.
Sean Martell, National Highways head of service delivery for the South East, said: “Our job is to keep the country moving and Operation Brock helps us to do that during particularly busy periods at the ports in Kent.”
He urged people heading to the continent over Easter to “allow yourself some extra time”.
He added: “We work hard to try and avoid disruption and delays, but these things can happen so always remember to carry essential supplies such as food and water and make sure your vehicle has plenty of fuel.”
Operation Brock involves narrow lanes and a reduced 50mph speed limit, leading to criticism from local leaders.
Meanwhile, the Port of Dover said it is part of a pilot to accelerate border procedures for school groups travelling by ferry to France.
From Thursday, school groups that submit passenger information to their ferry operator at least 72 hours before travel can benefit from faster processing during French border checks in Dover.
Port of Dover chief executive Doug Bannister said: “This pilot is a significant step forward in making school travel between the UK and France more seamless and accessible.
“By streamlining border processes and enabling visa-free travel for eligible students, we are reducing significant administration time, which in turn ensures a smoother, more efficient experience for school groups and potentially reduces queue times for additional tourist and freight traffic.”
Passengers were stuck on coaches for more than 10 hours at the start of the Easter school holiday period in 2023.
This was attributed to delays in processing passengers as French border officials carried out extra checks and stamped UK passports following Brexit.
The RAC is predicting more than 21 million leisure journeys will be made by car in the UK between Thursday and Monday, leading to delays on routes popular with holidaymakers.
Getaway traffic is expected to reach a peak on Friday and Saturday.
