Car use imposes costly environmental externalities. We investigate to what extent car trips could be shifted to low-emission modes, avoided via teleworking, or improved via a transition to electric vehicles in the context of daily mobility in the Paris area. We derive counterfactual travel times for 45,000 car trips from a representative transport survey, and formulate modal shift scenarios including a maximum acceptable increase in travel time. For a daily travel time increase below 10 min, 46% of drivers could shift to e-bike – mostly – or public transit – rarely –, with half of them benefiting from a travel time decrease. Such modal shift would reduce daily mobility emissions by 15% and generate annual climate and health benefits worth €125 million. Factors such as living in the far suburbs, being male, or having a high income, are associated with inability to shift modes. Teleworking two days a week could save an additional 5% of emissions. Holding demand for mobility and public transport infrastructure fixed, greater emission reductions require improving cars' environmental performance via a transition to electric vehicles.