We think is a minor glitch in FSX, which manifests or not, depending how much time it takes to load the scenery objects. Basically, it means the 3d highway should be loaded before the cars start to run, but it's not something that we can easily control.
There is a workaround, if you move the road traffic complexity slider to 0, return to flight, change it back to the value it was before, the road traffic will "reset" itself, and start going up on the highways.
We'll try to see if there's a way to fix this but, the fact that didn't happened at KLAS, might be just coincidental.
