The Road is an achievement for Halo 3 and Halo: The Master Chief Collection. In the original game, it is worth 20 gamerpoints and is unlocked by beating Tsavo Highway on Normal, Heroic, or Legendary. In The Master Chief Collection it is only worth 10 gamerpoints, though it can also be unlocked on Easy.
In the original game it is represented by a blue plate with a roman numeral III on it. In The Master Chief Collection it is represented by a screenshot of the level.
Note: You must finish the level completely through from Mission Start, otherwise it won't count.