The Carjacking medal (or Hijacker in Halo 3) is awarded for boarding an occupied vehicle from an opponent. It is available in all multiplayer gametypes where a vehicle is available. It has a Bungie difficulty rating of 2 out of 10.
This achievement is represented by a crossing sign with a U-turn sign on it.
After you steal an occupied vehicle, a message appears on your screen. In Halo 2, that message reads: "You boarded an enemy vehicle", while in Halo 3, it reads: "You hijacked (player's name)". Also, if you hijack and commandeer a person on the back of a Warthog who is facing away from you, you will also get an Assassination medal and the person will almost instantly die. However should they be facing you, they will only lose their shields and be stunned as usual. If you happen to get the Maybe Next Time Buddy Achievement, you will also receive this medal.