A traffic problem. The flow of traffic at a certain street crossing is described by saying that the probability of a car passing during any given second is a con- stant p; and that there is no interaction between the passing of cars at different seconds. Treating seconds as indivisible time units, the model of Bernoulli trials applies. Suppose that a pedestrian can cross the street only if no car is to pass during the next three seconds. Find the probability that the pedestrian has to wait for exactly k = 0, 1, 2, 3, 4 seconds. (The corresponding general formulas are not obvious and will be derived in connection with the theory of success runs in XIII, 7.)