Our brains are good...too good at times...at sorting sensory information into "important" and "unimportant". It would be overload to have to consciously notice every tiny detail. This is why my four year old son can spot the hidden stuffed monkey at Trader Joe's at an instant while I never noticed it was ever there. Unfortunately there's a lot of drivers out there whose brains subconsciously put bikes into the "unimportant" category, which means that they see them, but their brain filters that information from being considered in decision making. You might as well be a mailbox or a garbage can on the side of the road. Also I think that explains much of the irrational anger that erupts at cyclists. There's a cognitive dissonance between what is (a cyclist who they have to adjust their driving for) and what their subconscious preconception is (that they should only have to adjust for other motor vehicles). They aren't angry at the cyclist, really. They are mad that the world suddenly doesn't make sense, like old people who grew up in segregated society having to deal with integration and civil rights.