May sound a silly question,but how how do they know how long you've traveled on that particular road? you may have joined the route 5 miles before the toll or you could have been on it for 50 miles. Is it just a set charge to pass through that particular barrier regardless of how far you've traveled before it or intend to travel beyond it?
Don't forget to leave a review of the French and other European campsites you have visited!
From what I have picked up so far,you pull a ticket on entry to the toll road, and pay as you exit using the same ticket,same as automated car parks.Except on certain fixed charge roads where you pay as you enter a section!