Optimizing metro and BRT transfer design in Isfahan using passenger generalized cost and ant colony optimization

This study presents a revised framework for screening and optimizing local Metro and BRT transfer connections in Isfahan using a state-separated passenger network, fixed-OD evaluation, explicit transfer screening, and ant-colony-based search. The formulation prevents implicit intermodal switching and holds the OD matrix constant between baseline and intervention networks, thereby separating modeled network-design effects from demand regeneration. Because the available empirical demand information does not provide a complete observed spatial OD matrix, the gravity deterrence coefficient is treated as a sensitivity parameter rather than as an empirically calibrated behavioral coefficient. Coordinate and network screening retained 29 feasible transfer candidates. Candidate 22, a 249.2 m pedestrian connection, produced the largest modeled improvement in the central case. System passenger generalized cost decreased from 6,265,519.636 to 6,258,241.797 passenger-min, a reduction of 7,277.839 passenger-min (0.116157%). Exhaustive evaluation of all 406 candidate pairs and targeted triple screening found no additional benefit beyond the best single link. I-ACO, standard ACO, and greedy search identified the same one-link solution, although the fast benchmark does not establish statistical superiority of any stochastic algorithm. Given the small system-wide effect and limitations in observed OD validation, route-choice behavior, and service-parameter calibration, the result is interpreted as a localized model-based screening outcome that requires empirical and site-level validation before implementation.