Endogenous Modal Shift and Technology Change in Spanish Passenger Transport: A System Dynamics Scenario Analysis to 2050

The decarbonization of passenger transport remains one of the most challenging sectors for climate policy, as supply-side technological solutions alone are insufficient to meet stringent climate targets. This paper presents an extended system dynamics model of the Spanish passenger transport sector, built on the WILIAM integrated assessment framework, that for the first time endogenizes modal shift, allowing transport mode shares to emerge from the interaction of policy instruments, price signals, and infrastructure provision. Four scenarios are evaluated to 2050: a Baseline, a scenario calibrated to the Spanish National Integrated Energy and Climate Plan (GG-PNIEC), a more ambitious electrification and modal-shift scenario (GG+), and a Decent Living Standards scenario (DLS) combining technological change with deep demand reduction. Relative to 2023, direct CO2 emissions in 2050 fall by 64% under GG-PNIEC, 75% under GG+, and 84% under DLS. While GG-PNIEC and GG+ deliver substantial reductions through electrification, only DLS approaches deep decarbonization, combining rapid electrification with a reduction of over 55% in light-duty vehicle travel demand relative to the Baseline. These results indicate that technological substitution alone is insufficient, and that sufficiency-oriented demand reduction is decisive for reaching the deepest emission cuts in passenger transport.