Just think of all those people driving around. You know the ones. They veer dangerously into your lane as they negotiate corners a little too broadly. They stare, stupefied at green lights, then suddenly slam on the accelerator, leaving you at the read light. They drift slowly over into your lane without using a turn signal, and without seeing you there. They careen wildly away from the bar, where they just polished off a pitcher of Guinness.
Now, think of them in 3 dimensions.