My own settings really are not very useful to discuss, because I keep changing them depending what I'm doing. I value fps above everything else, so my own preference of settings (IF I wanted to just *use* FSX like an user, which is something I don't do very often) it's usually much lower than what most of users have, since my system is optimized for development, not flying, (it's an MacPro 8 core Xeon with 2.66 mhz clock, so it's not very fast for FSX, but it's able to keep many programs open at the same time without much slowdown). So, I usually have most of the traffic turned entirely down, and I turn on AI traffic only for testing.
In any case, road traffic changes a lot depending on the area, because its density changes both depending on the kind of road (highways are much denser) and also the population density in that area, which means any comparison should be made in the same place and even using the same installed sceneries, because a scenery can change the road layout and this will affect the road traffic density too.
Yes, as with any other setting with a large variation in objects density, the road traffic has an impact, but it doesn't seem as bad as other settings, like AI Traffic or Autogen but again, this is changing depending on the area/scenery used, because on an airport with 10 parking stands in the middle of the desert, you can use 100% AI Traffic and Autogen to the maximum and it won't affect fps at all, while a large airport with 150 parking stands close to a dense city, will bring any system down to its knees with the same setting.
