A high proportion of car trips can be replaced by a combination of public transit and cycling for the first-and-last mile. This paper estimates the potential for cycling combined with public transit as a substitute for car trips in the Lisbon metropolitan area and assesses its socio-environmental impacts using open data and open source tools. A decision support tool that facilitates the design and development of a metropolitan cycling network was developed (biclaR). The social and environmental impacts were assessed using Health World Organization tools. The impacts of shifting car trips to public transport were also estimated and monetized. The results show that 10~% of all trips could be made by cycling in combination with public transport. Shifting to cycling for the shorter first and last mile stages can reduce annual CO2eq emissions from 3000 to 7500 tons/day, while for the public transport leg, the transfer from car avoids of up to 20,500 tons of CO2eq emissions per year. The estimated socio-environmental benefits are of €125 million to €325 million over 10~years. This evidence can support policymakers to prioritize interventions that reduce the reliance on private motor vehicles.