UC Irvine Institute of Transportation Studies

At present most patronage predictions of transit systems are performed using UMTA's UTPS package or some adaptation of it. The transit assignment produced by a typical UTPS system can be classified as an All-or-Nothing limited equilibrium assignment. However, passenger loads assigned to a transit line can far exceed the line capacity. In such a case, line headway has to be reduced to provide enough capacity to accommodate transit demand. If the increase in frequency is not accounted for by iterating again through the mode choice and assignment models, the equilibrium assumptions are violated. If equilibrium between demand and supply is achieved it might occur at a point which requires transit capacity much beyond the economically feasible or engineering practical level. Thus the present transit assignment procedure suffers from two problems. First, trips are assigned to transit lines with disregard to their actual capacity. Second, while some lines are assigned passenger loads beyond capacity, there might be other lines with just slightly longer travel times which are greatly underutilized. A realistic assignment should take into account and not exceed the actual capacity of every transit line. Furthermore, it should consider lines capacities while rationally simulating people's travel behavior. In this paper a transit assignment algorithm is presented which takes into account the actual capacity of transit lines and assigns trips to more than a single path when the shortest path reaches its capacity. This procedure produces a practical Multipath Capacity Limited Transit Assignment (McLAT). The procedure was implemented on an IBM mainframe computer using standard UMTA's UTPS package with the addition of only one Fortran program.