Diagnosing public transport disadvantage in multi-activity accessibility: Opportunity-side and service-side signals

Multi-activity daily travel requires access to several everyday opportunities within limited time budgets. Public transport (PT), however, often provides less effective door-to-door access to such opportunity combinations than private cars. Using scheduled PT and private-car travel times for access to eight categories of essential facilities across 229 South Korean city-level administrative units, this study develops a diagnostic framework that distinguishes facility-side reachability signals, weak co-location-based bundling signals, and PT-side penalty signals. First, under the national population-weighted mean reference lines used for the baseline classification, 114 of 229 units (49.8%) fall into the compound {F(s), F(d), T(c)} regime, in which all three signals co-occur. Second, an aging-related vulnerability pattern emerges: older-adult share is positively associated with all three diagnostic signals under the mean-based baseline, identifying units where aging-related vulnerability and potential-accessibility constraints warrant closer equity-focused assessment. Third, weak opportunity structure is the clearest common correlate across diagnostic signals, suggesting that PT disadvantage should be assessed alongside facility distribution and activity co-location, not only transit service conditions. By translating PT–car accessibility gaps into diagnostic signals, the framework indicates transport and land-use domains that warrant joint assessment before intervention design.