At present most patronage predictions of transit systems are performed using umta transportation planning system (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 againthrough 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 that requires transit capacity much beyond economically feasible or engineering practical levels. Thusthe present transit assignment procedure suffers from two problems.First, trips are assigned to transit lines without regard 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 that 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. A transit assignment algorithm is presented that 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 the standard utps package with the addition of only one fortran program. This paper appears in transportation research record no. 1283, Transportation systems planning and applications 1990.
Samenvatting