Ghost Drivers are invisible NPCs that may sometimes spawn in a moving, and operating vehicle, due to a glitch. They can be witnessed in GTA IV Multiplayer and GTA Online the most. Rarely do they spawn in single player. The vehicles driven by ghost drivers appear to have no visible civilian driving, and they do not act whatsoever to a player's presence. If you shoot at them, they do not enter panic mode. Sometimes, the player cannot steal the vehicle, even when the windows are shot completely out.
GTA IV Multiplayer
These are uncommon in normal, unmodded free roam lobbies. They cannot have their vehicles stolen, and can kill the player involuntarily. In modded lobbies, they can be seen quite a bit.
Ghost Drivers normally spawn as docile entities, not trying to kill the player, with the exceptionally rare case of running a player over without hesitation. If a server has quite a few people in it (8+), they can be seen randomly driving, and stop at the player, if standing in its way. The player can steal this vehicle like an unlocked parked car, but no hot-wiring, or carjacking animation plays. Sometimes, the player freezes in mid-action entering the vehicle. On some occasions, before patch 1.0 and 1.1, a nearby player entering a ghost driver's vehicle can cause every vehicle with a 2-mile radius to despawn. This sometimes glitches, and can make rare cars spawn frequently, though it is rare.
On some occasions, deadly drivers may be armed and attempt to shoot at players, as if they were griefing players themselves. A possible explanation could be that they are just criminals that would normally engage with the L.S.P.D. By chance, the player might manage to engage these criminals, before the police does so. Another explanation could be that the hostile NPCs are actually other characters of players that failed to completely join the session.
Possible Explanations for Ghost Drivers
It could be a conflict of spawning a parked car and an NPC driver at the same time, creating a driving parked car of some sort. It also could be a fail-safe in the game that prevents modders from spawning more cars that can be entered. Another possibility is severe server spawning lag, where the drivers don't spawn whatsoever. The factors in the game that cause these strange glitches, are not known. In the 3D Universe, drivers will cause them to panic and become more aggressive than the HD Universe.