We have now completed our tour of derived distributions, geometrical probability, and spatial processes. For those interested in further study of geometrical probability, we recommend the recent book by Solomon [SOLO 78]. Chapter 4 switches emphasis from space to time, dealing with congestion that arises in queueing systems. Later chapters rely heavily on the probabilistic modeling methods of Chapter 2, this chapter, and Chapter 4, to study congestion phenomena in a spatially oriented urban setting.