Sustainable Passenger Transport in Spain: A System Dynamics Assessment of Endogenous Responses and Non-Additive Policy Interactions to 2050

Sustainable mobility requires reducing energy use and emissions while considering accessibility and pressure on low-carbon resources. This study assesses passenger transport in Spain to 2050 with a system-dynamics model linking demand, modal and powertrain choice, fleet turnover, charging and fuel origins. Choice equations are estimated on pre-COVID data, with specification selection using 2001–2014 and evaluation with 2015–2019, followed by estimation with 2001–2019. Post-2019 observations provide diagnostic comparisons. Stylized Avoid–Shift–Improve packages share a common policy-reference approximation. In 2050, REF reaches 797.1 Gpkm, 754.9 PJ and 22.46 Mt of attributed operational CO2. Avoid, Shift, medium Improve and high Improve reduce this emissions indicator by 9.87%, 0.39%, 2.07% and 3.94%, respectively. Both integrated packages are subadditive at the endpoint, while medium ASI is superadditive cumulatively. Parameter, policy-intensity, regulatory, resource-cap and fleet-allocation sensitivities qualify these comparisons. Uncapped high ASI with Fossil-Free fuels approaches zero attributed operational CO2, but high resource caps raise its 2050 value to 29.49 Mt. Physical combustion remains positive. These conditional experiments identify energy, climate and resource trade-offs; they do not establish life-cycle neutrality, equitable accessibility or probabilistic forecasts.