Let be the mth moment of the sojourn time distribution for some particular customer class in an open queueing system, where is the overall arrival rate. The paper derives closed form expressions, in terms of the basic system data, for the first order light traffic limit, , in the cases where the total arrival process is Poisson, a phase-type renewal process, and a superposition of independent phase-type renewal processes. For certain phase-type renewal processes, the nth order light traffic limit is zero for . In these cases the th order limit, is derived. The expressions are numerically tractable. The most difficult operation is a matrix inversion.
