Urban Emergency Service Systems

An Iterative Procedure for Approximating Performance Characteristics

by Richard C. Larson

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Presentation of a simple procedure that can compute by approximation all performance characteristics of an exact model of an urban emergency service system. The model assumes a Poisson distribution of service calls with an M/M/N queuing system of either (1) zero-line capacity, or (2) infinite-line capacity. For (1), calls arriving while all units are busy are lost or serviced from outside or by special reserve units; in (2) such calls are entered at the end of a queue that is depleted in a first-come, first-serve manner. Calculation for this approximation method can often be performed manually with the aid of an electronic calculator. For most performance measures the values estimated by this procedure are within two percent of the exact values. Since the accuracy of the approximation does not degenerate with increasing numbers of units, this technique would be preferable for emergency systems having more than 15 cooperating units.

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