It is a common belief that public transit is a more environmentally-friendly way to get from place to place than driving a car. However, it is not that straight-forward. Depending on the situation, public transit (specifically buses) can cause either less or more harm to the environment than passenger vehicles. Using this simple calculator, you can calculate how many people must ride the bus in order for it to save on fuel usage, and therefore save the environment, compared to driving a car. The way this calculator works is by comparing the amount of fuel consumed per person transported. For a given distance travelled, if two vehicles carrying one person each consume different amounts of fuel, then the vehicle that consumed the least amount of fuel caused its occupant to contribute the least to CO2 (and other pollutant) emissions. If a vehicle contains more than 1 occupant, however, the carbon footprint is divided between the two or more occupants. It is important to note that adding more passengers to a vehicle does not significantly increase the vehicle's fuel consumption, since human bodies weigh relativity little in comparison to the mass of a car. This is why carpooling is recommended to reduce the overall emissions of society. This principle of spreading the carbon footprint created by a car across multiple people is what allows a large vehicle like a bus to contribute less to pollution than cars, provided that it is carrying enough people to create less emissions than the cars those people would have otherwise driven would create. Using this knowledge, the calculator takes in values for the fuel consumption of the average passenger car and average city bus, as well as the average car occupancy rate (average amount of people per car), and uses a simple formula to output the amount of occupants required in a city bus (rounded up to the nearest person; not including the bus driver) in order for less pollution to be created than if those occupants drove their own cars instead. Using the on-screen sliders, you can vary the input values to see how it affects the amount of passengers a bus must contain; different regions around the world have different vehicle fuel efficiencies and car occupancy rates.
Source for car occupancy rate: https://oee.nrcan.gc.ca/publications/statistics/cvs/2009/chapter2.cfm