Select zone analysis examines the spatial impacts of a new development and requires knowledge of the distribution of path flows or of the distribution of origin–destination (O-D) specific link flows. Because these two flow distributions may not be uniquely determined from assignment of deterministic user equilibriums, the choice of a particular flow distribution as the basis for select zone analysis remains an open question. This paper both suggests the use of the mean of all the solutions for path or O-D–specific user equilibriums as that basis and proves its stability. A sampling procedure is proposed to estimate the basis. Numerical examples are provided to demonstrate the proposed concept and compare it with the entropy maximization approach.