I think your first response is right, if you compare the roads to similarly-sized examples in LS there is still a big difference in traffic.
Edit: I went back and looked at the n00bs/pros plot and concentrated on this sculpture in SF:
Apparently pros pass it on the East side, and n00bs pass it on its West side...quite interesting lol.
Edit2:
Here is a 10x10px histogram overlayed to demonstrate the most popular areas of the map:
Clearly LV is significantly less popular.