Wow, I'd really like to get a hold of some of whatever is being smoked around here, the AI is unbelievably bad in GT5, only marginally improved from GT4 as far as not running into you, i.e. they don't usually plow right into you all the way down a mile-long straight anymore. They still smash right into you in the corners when you are fully alongside them or even slightly in front of them when entering the corner. They still slam hard into your rear end if you slow down a tiny bit earlier than they think you should for a corner. They still slam into you mid-corner if you take it slow-in and fast-out if they want to take it fast-in and slow-out(which seems their preferred method).
They've had 13 years to improve the AI and they still have not managed to make them remotely competent at driving around ONE track in all that time, even the tracks that have been around forever. They take the wrong line through half the corners, slow down early then hug the very inside all the way through and then accelerate too early and have to slow down before they hit the wall(or they think they will hit the wall when they won't).
They will NEVER EVER make any attempt whatsoever to block you, you can do whatever you wish and they will not react to what you are setting up behind them. If you are going fast enough they will always leave plenty of room for you to get alongside them going into a corner. The only time they leave their line is when they are attempting to make an overtaking maneuver on a car in front of them -- usually too far in front for their maneuver to have a chance of success. Sometimes they give the impression of trying to block because they leave lots of room into a corner and then slam the door shut as they try to take their normal racing line through the corner with no comprehension whatsoever that there is a car trying to squeeze into the gap they leave, or because as I just stated they'll move off line to try to pass a car 50 feet ahead of them, but that's about it and they aren't doing it intentionally.
They do ATTEMPT to move out of your way when you are lapping them, which is a nice thought, but fail completely and utterly at doing so. When you are 500 yards behind on a straight, they will constantly weave back and forth to 'get out of your way' even though you are nowhere near being close enough to do anything other than think about possibly attempting a pass half a lap later. It gives the impression that they are actually trying to break the draft. When you do get close they will almost always attempt to get out of your way by jumping off of the racing line to the INSIDE of the next corner, making getting past them as difficult as possible, complicated by the fact that they slow way down as you get close, so as you are setting up a nice smooth pass on a lap car, they will jump right in front of you and then slam on the brakes and hog the shortest way through the corner, and then repeat it on the next one. It would be FAR more helpful if they would just completely ignore the fact that you are laps ahead of them and just act normally.
Lastly, I think it is a problem with triggering this lappery behavior that has completely taken away any possibility of having a race with the AI since the recent round of patching. They don't necessarily move out of the way, but in the heat of battle something triggers their impulse to slam on the brakes and slow way down to let you past, not the same as the slight lift they do when you gently(or not so gently) tap them(which is also stupid). This behavior can be easily triggered any time you start to overlap them while they are turning. Sometimes it will even happen when you are directly behind but within a foot or two of them, they'll just stop for no reason(where they otherwise never would) and ram into you with their rear bumper. So basically any time you need to get by a car, you just pull alongside by a few inches, generally easiest to do by going to the outside, and they'll just stop and say "sorry sir, go on through." They'll even do this in flat out sections after a corner, as long as they are still turning a slight bit. In order to actually have a battle with them you need to keep this in mind and not do things that will make them stop. If you pull alongside while on a straight, they will attempt to stay with you and hold the position through the corner, but if they get 2 feet in front and then you close those 2 feet back while still turning, they'll park.
*exhales* OK, rant over.
