Road layout determines driving behavior independent of traffic density

You could say that driving behavior consists of speeding up, slowing down, and changing lanes. I note - while driving past Route 20 going south on 128 at 1:30PM Friday - that people are making the same lane changes that, later on, gunk up the traffic, but which do not at the lower density. The behavior is still there but it does not affect traffic at this density.
On the other hand, speed changes and opportunistic lane changes (the ones that aren't necessary) are related to density.