Urban congestion is difficult to alleviate through road expansion alone, making it necessary to improve existing road-network performance while maintaining public-transport priority. Under transit-priority policies, buses and private vehicles share limited road space, and bus lanes, stops, and lane-changing interactions may either improve or reduce network efficiency. This study treats capacity improvement as a constrained network-performance objective that preserves feasible bus operation and supports higher-occupancy, lower-emission public transport. A simulation-based capacity evaluation framework was developed using the three-dimensional macroscopic fundamental diagram (3D-MFD). A grid network was built in Simulation of Urban Mobility (SUMO), and 16 scenarios were designed by varying four factors: dedicated bus-lane proportion, average bus dwell time, driver lane-changing willingness, and bus-to-private-vehicle ratio. The 3D-MFD was fitted by nonlinear least squares, and range analysis and analysis of variance were used to identify significant factors. Results show that bus-lane proportion and bus-to-private-vehicle ratio dominate capacity variation. A supplementary simulation-based transferability assessment on a Beijing subnetwork further showed that the 4% bus-lane share and 7% bus-to-private-vehicle ratio produced the highest tested capacity response. The findings provide an assumption-bounded basis for screening bus-priority parameters rather than universal design values.