I understand about the FS9 issue, but can you create custom traffic in FSX?
I'm not sure what do you mean with "custom" traffic here.
If you mean being able to have cars following a custom path which can be specified to conform to the airport, then it's an FSX feature (not available in FS9), which we used in most of our sceneries already.
This is quite convenient, both because it can be user controlled in complexity (or turned off) very easily using the FSX settings, and because it's much more fps friendly than FS9-style custom animations. Another advantage of this, is a better continuity between the car traffic flow at the airport, and the rest of the surrounding scenery, which you might have enhanced with other products, like Ultimate Terrain or similar ones.
If, instead, you mean cars with custom appearance, then it's a plain 3DS animation that, even if it allows to customize the cars appearance, doesn't allow for much density without killing the performances, and doesn't really integrate with the rest of the scenery. That's what we used to do road traffic in FS9, but doesn't really make much sense in FSX.
For example Orbx Melbourne has road traffic coming in and out of the airport entrance, as well as roads near the airport. Will you guys be able to create this feature for LAX?
We had road traffic in FSX for ages...at KFLL, KLAS, PHNL and KDFW, and it also follows the elevated road bridges around the airport, so there's are no reasons to think they wouldn't be at KLAX as well.