Chris Butcher has posted a message in the HBO forum explaining that the artificial intelligence in Halo can drive all the vehicles except the warthog . He explains:

if you think about it, it's much harder to write AI that can handle turning angles, sliding around corners, the danger of flipping while cornering at high speeds and things like that. another big problem is that if the jeep comes to a halt somewhere, it may require you to back up and execute a three-point turn or something in order to head in a new direction. that requires significant changes to the movement logic (which ordinarily assumes that a driver can move in any direction at any time).

Butcher goes on to say that it was hysterical watching Ferrex and Ruiner's AI try to operate the Warthog, but overall it wasn't solid enough to rely upon .