Metro 3 in Brussels under uncertainty: scenario-based public transport accessibility analysis

Metro Line 3 in Brussels is one of Europe's most debated infrastructure projects, marked by escalating costs, delays, and uncertainty over completion. Yet no public accessibility appraisal exists to inform this policy debate. This paper provides a scenario-based assessment of the spatial and distributional accessibility impacts of Metro 3 using schedule-based public transport data. Official GTFS feeds from all regional operators were combined and adapted to represent three scenarios: (i) the current network, (ii) partial implementation of Metro 3 (conversion of the southern premetro section), and (iii) full implementation to Bordet. Accessibility was measured as public transport travel time between 647 evenly spaced 500 m points covering the Brussels-Capital Region. Simulations were conducted for morning, evening, and weekend midday periods, each with three departure times (t-10, t, t+10), capturing robustness to short-term timetable variation. Results show substantial but uneven accessibility gains, with the largest improvements occurring in neighborhoods with below-average incomes. Temporal robustness analysis highlights variability in accessibility outcomes, underscoring the need to account for uncertainty in departure timing. These findings suggest that Metro 3 has the potential to reduce socio-spatial inequalities in accessibility, providing transparent evidence for a project where public debate is dominated by cost concerns.