"R is a velocity measure, defined as a reasonable speed of travel that is consistent with health, mental well-being and not being more than, say, five minutes late. It is therefore clearly an almost infinite variable figure according to circumstances, since the first two factors vary not only with speed taken as an absolute, but also with awareness of the third factor. Unless handled with tranquillity this equation can result in considerable stress, ulcers and even death.

"R17 is not a fixed velocity, but it is clearly far too fast."

     - The Hitchhiker's Guide To The Galaxy