Your personal action affects only your own carbon footprint

That's not true. If I take the bus, I increase ridership and resources for buses, which in aggregate can lead to improvements to the bus route, which can convince others to ride. the people at your local city government can have a much easier time justifying an increase to public transit spending if they can show high or increasing transit ridership. Depending on your individual circumstances one may provide better impact to effort than another but taking a bus is as much political action as voting.