From Halopedia, the Halo wiki
Rocket Race is a recurring game variant in the Halo series. In Rocket Race modes, teams drive indestructible vehicles around a map in an attempt to reach goal zones first; passengers in vehicles are typically armed with Rocket Launchers, and can use these to launch enemy vehicles away from goal zones. In Halo 3, the game variant was based on VIP; in Halo: Reach, it was based on Rally; and in Halo 4, it was based on King of the Hill.
In Halo 3 Rocket Race matches, four teams of two players competed to reach as many goal areas as possible. At the start of a match, a player on each team would be randomly given VIP status; they would ride in the passenger seat of a Mongoose, while their teammate would drive them around. VIPs were incapable of dealing damage, but could kill other players through assassinations or by luring them into map obstacles and level boundaries.
Rocket Race was initially available in its own playlist, but the playlist was retired when the game's second Title Update was released. Rocket Race remains available in Multi-Team, with a low priority.
Rocket Race was introduced to Halo: Reach Matchmaking upon release of Noble Map Pack, which also includes one related achievement: Offensive Driver, awarded for killing a player in Rocket Race. The game mode was a modified Rally variant, in which players on each team were forced into fixed "driver" and "gunner" roles. The default settings mimicked the Halo 3 setup, giving players Mongooses and Rocket Launchers. In addition to the standard Rally settings, Rocket Race offered a new set of features:
Rocket Race was later replaced by Rocket Hog Race, in which players drove Rocket Warthogs and used both Rocket Launchers and Grenade Launchers. Whereas a Rocket Launcher could be used to send an enemy vehicle flying, the Grenade Launcher could be used to EMP enemies and stop them in their tracks. In addition to these changes, Rocket Hog Race no longer forced players into fixed roles; instead of offering Driver and Gunner Traits, the game variant offered In Vehicle Traits and On Foot Traits. (Accordingly, players were invulnerable in vehicles, but vulnerable on foot; and players on foot were marked with waypoints that were visible to their enemies.)
On February 8, 2011, Rocket Hog Race was updated to allow players in Custom Games to choose whether to use Mongooses or Rocket Warthogs.
Rocket Race and Rocket Hog Race matches were available in the Multi Team playlist, and were typically played on Hemorrhage.
A Halo 4 variant was introduced as a temporary playlist available between December 23, 2013 and January 5, 2014.