If you're going down the Rhone Valley via Lyon then weekends will be very busy. Other routes are usually fine at weekends until you hit the A7 / A8 / A9 motorways that run along parallel to the south coast.
Midweek is usually no problem.
Which part of the south of France are you aiming for - the southern coastline is over 300 miles long.
We travelled to the Ardeche from Calais on the main Autoroute to the south (via Dijon, Lyon etc) on the 3rd August and the traffic was fine, a bit busy in Lyon, but otherwise ok.
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May 18, Delftse Hout, Delft
Aug 18, Le Logis du Breuil, Royan
Usually I would say avoid the middle weekend. Generally around the 17th August as its about the same time as the bank holiday. Roads are very busy that weekend.
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Thank you for all the replies. We are looking to drive from Calais to Braize for a week, then head down to Vias Plage for a couple of weeks and then head up to the Ardeche for a week.
We can head to France slightly earlier next year as we break up from work on the 17 July so we are probably going to drive down to Dover on the 26 July, stay overnight and then catch the ferry in the morning and drive to a Braize on the Monday. That would mean then driving to the Ardeche from vias Plage on monday 17 August....