It is clear that if we look at the first chapter of Book the Third, entitled "In Secret," we find that Darnay experienced many problems during his journey to reach Paris to try and save the life of the faithful Gabelle. Clearly, Dickens presents to us a picture of France in chaos, where the established order of things has been overturned and has vanished, and therefore it is very difficult for somebody such as Darnay to travel unmolested and unhindered. Note what the text tells us in this chapter about his journey:
This universal watchfulness not only stopped him on the highway twenty times in a stage, but retarded his progress twenty times in a day, by riding after him and taking him back, riding before him and stopping him by anticipation, riding with him and keeping him in charge. He had been days upon his journey in France alone, when he went to bed tired out, in a little town on the high road, still along way from Paris.
We are told that it was only the production of Gabelle's letter that had managed to get him this far, and he had been forced to use the letter again to pass the guard house that had held him for so long. The atmosphere of approaching terror and doom thus is created, which prepares us for Darnay's arrest and his escort to Paris.