The mender of roads, who first appears in book 2, chapter 8, is called Jacques by Defarge. He's from the peasant class and witnessed a hanging. He goes with the Defarge family to see the King and Queen of France.

In chapter 15, Defarge brings the mender of roads to the wine shop and takes him upstairs to tell his story to the others. He introduces the mender to his wife, saying:

"My wife," said Defarge aloud, addressing Madame Defarge: "I have travelled certain leagues with this good mender of roads, called Jacques. I met him—by accident—a day and half's journey out of Paris. He is a good child, this mender of roads, called Jacques. Give him to drink, my wife!"

According to the mender, he saw a man hanging from a chain under the carriage of the Marquis. When asked about the man, he says only that he was tall and like a spectre. He says he later saw the man bound and left in a cage until he was hanged despite petitions asking to spare him. The Defarges and the other listeners—also each called Jacques—listen intently to the story.

The mender of roads is a “grizzled” and poor man who finds a way to make himself useful to the revolution by relating the story of the man under the Marquis’s carriage.

The mender of roads becomes popular overnight when he notices the man under the carriage and tells the Marquis.

Monsieur the Marquis cast his eyes over the submissive faces that drooped before him, as the like of himself had drooped before Monseigneur of the Court—only the difference was, that these faces drooped merely to suffer‚ and not to propitiate—when a grizzled mender of the roads joined the group. (Book 2, Ch 8, p. 73)

The mender of roads sees a man hanging from the Marquis’s carriage.  We track his progress as he sees the events unfold and then goes to tell the Defarges.  He is known for his blue cap.

It was high noontide, when two dusty men passed through his streets and under his swinging lamps: of whom, one was Monsieur Defarge: the other a mender of roads in a blue cap. (ch 15, p. 107)

The mender of roads is young, and seems to enjoy his fifteen minutes of fame.

Again the mender of roads went through the whole performance; in which he ought to have been perfect by that time, seeing that it had been the infallible resource and indispensable entertainment of his village during a whole year. (p. 108)

The mender of roads is used by Dickens to demonstrate to the reader the progression of the revolution and its impact on people who were involved peripherally.