Quick Answer — It Depends on Distance, Fare, and Your Car's MPG
Public transit's cost and emissions advantage over driving isn't automatic — it depends on your specific commute distance, local fare, and how fuel-efficient your car already is. This calculator compares annual cost (transit fare vs. gas plus per-mile operating cost) and annual CO2 emissions (real-world average bus or rail per-passenger-mile emissions vs. your car's fuel economy) side by side.
For emissions, this calculator uses real-world average figures: buses average about 431 grams CO2 per passenger-mile and rail averages about 77 grams CO2 per passenger-mile, both reflecting typical US ridership levels (a bus or train's emissions get divided across however many passengers are actually riding — a mostly-empty bus can have a worse per-passenger footprint than a fuel-efficient car). Your car's emissions are calculated from EPA's gasoline combustion factor of 8,887 grams CO2 per gallon divided by its miles-per-gallon rating.