"So the laws of good driving forbade you to go off the magic ribbon except in extreme emergencies. You were ethically entitled to several inches of margin at the right-hand edge; and the man approaching you was entitled to an equal number of inches; which left a remainder of inches between the two projectiles as they shot by. It sounds risky as one tells it, but the heavens are run on the basis of similar calculations, and while collisions do happen, they leave time enough in between for universes to be formed, and successful careers conducted by men of affairs."