Never had it happen to me either, or seen it take place. What I will say though if it's an organised rally or you are going to Le Mans, the cops are all over the toll booth areas like a rash. Mainly, they hide under bushes or on top of flyovers, with radar guns and radio ahead with car descriptions, then they have you at the toll booth as you have to stop, so no chase or drama for them.
I prefer to stick to non toll roads when possible, if I do use tolls, you can still rack up a fair pace and avoid the Gendarme's.