Traffic Capacity of a Probability-Engineered Truck Group

The grade of service for a probability-engineered trunk group is defined to be the average blocking observed in the time-consistent busy hour of the busy season.* The existing methods for predicting grade of service do not account for the effects of the finite length of the individual one-hour measurement intervals and thereby tend to underestimate trunk-group capacity. In this paper, we develop an improved model for calculating average blocking that includes the two essential effects of the finite measurement interval. First, the current method for estimating the mean blocking for a single hour must be revised to remove an implicit assumption that the measurement interval is infinite. Second, the existing mathematical model for day-to-day variation of trunk-group * Both the C C I T T (International Telegraph and Telephone Consultative Committee) and the Bell System define grade of service as an unweighted average of busy-hour busy-season blocking values. 831 offered loads must be modified to account for statistical measurement error, which is also introduced by the finite measurement interval. We develop a new model of day-to-day load variation in Appendix A and in Section II combine it with a new estimate of mean single-hour blocking to obtain our approximation for the average blocking. This new approximation is then compared with the existing approximation analytically and numerically; the accuracy of the new approach is established in the third section using data from a computer simulation.