From Halopedia, the Halo wiki
Sunday Driver is a spree medal in Halo: Reach multiplayer, campaign and Firefight. It is awarded for splattering fifteen opponents without dying. A medal of the same name is awarded in Halo: Spartan Assault and Halo: Spartan Strike for killing ten enemies in a row, with a vehicle, without being hit.
In Halo: Reach it is represented by a silver six-pointed star with a Splatter medal symbol in the center, with a purple blood splatter rather than red. In Spartan Assault and Spartan Strike it is represented by a shield with three chevrons and steering wheel.