Disclaimer: Please refer to your local social distancing recommendations and guidelines in time of Covid19 pandemic with reference to public transportation.
Public transportation has the lowest CO2 footprint per passenger per km among all massively used transportation means. Of course, this depends a lot of the fuels used but economy of scale certainly plays an important role here. Besides some direct CO2 benefits there are also some others, like less space needed for car parking, less congested roads and others.
What can you do if your country has underdeveloped public transportation. Demand drives supply. Be active, one person cannot change much but a group can. Join here and promote your environmental friendly lifestyle. Publish your thoughts and appraise other for their achievements.