They aren't any more aggressive than normal..it's just that Nascar is very close and tight racing. The AI have their pre-programmed line to follow (on banked curves it is go in high and then come down to the inside) so if you are on their line, namely inside them, when they drop down they will hit you as they are not programmed to react to the player, only other AI racers.
For the ovals I just set my transmission to 10mph higher than standard which usually allows to to get in front then it's a matter of keeping them behind you. The last race is Indy right? (can't remember for sure as not got PS3 on) If so, then you can take much more speed into the corners than the AI so it is fairly simple keeping them behind you....just have to be careful when in the pack..
