For me it is about anticipation vs. reaction. With a sufficient degree of attentiveness and even an hint of spatial awareness, one should be able to observe far enough ahead to anticipate possible collisions and conflicts. And one should still be able to fully take in and process one's surroundings at the same time. Maybe it is an NJ think, a constant weeding out of things you are already familiar with (a series of stores all selling the same things, cars, people, etc. commonplace things that require no processing), so you can focus on and quickly analyse and absorb the very rare new data that might come along (i.e. a piano falling from a rooftop).