Public Transport Savings Calculator

The comparison hinges on distinguishing marginal from fixed costs of car ownership. Fuel is marginal, computed as annual miles divided by fuel economy multiplied by the pump price, and it is the only component most people mentally account for. Insurance premiums, registration, financing payments and depreciation are fixed: they accrue whether the car is driven 3,000 miles or 15,000, which is why cutting commute days by working remotely reduces car costs far less than proportionally. Depreciation is typically the single largest item, running 3,000 to 5,000 dollars per year on a newer vehicle since a car commonly loses 20 to 30 percent of value in year one and near 15 percent annually thereafter. The American Automobile Association has consistently found total ownership costs for a new vehicle above 10,000 dollars per year at 15,000 annual miles, which is several times the price of a typical transit pass. The emissions side uses different units on each leg. Car emissions follow directly from fuel burned, at 8.887 kg of CO2 per gallon of gasoline combusted. Transit emissions are expressed per passenger mile and depend entirely on vehicle occupancy: a single occupant car emits roughly 400 g per mile, a diesel bus at typical ridership around 90 g per passenger mile, and electrified light or heavy rail considerably less. A crowded bus is efficient and a nearly empty one is not, so system-average factors conceal wide route-level variation. Two caveats: this model excludes depreciation from the headline saving, and the saving only reaches its full value if the household actually sheds a vehicle rather than parking an owned car during the week.

The data infrastructure behind this comparison was built by two separate programs. On the vehicle side, the American Automobile Association has published its Your Driving Costs study since 1950, establishing the convention of expressing ownership as cents per mile across fuel, maintenance, tyres, insurance, licence fees, depreciation and finance charges, and it remains the reference series that most cost comparisons cite. On the transit side, the National Transit Database was mandated by the 1974 amendments to the Urban Mass Transportation Act, requiring federally funded agencies to report standardised operating, ridership and cost statistics, which is what makes passenger mile and per-trip figures comparable across systems. The explicit savings framing is more recent. The American Public Transportation Association began publishing a regular Transit Savings Report in the 2000s, contrasting the cost of a monthly pass in major metropolitan areas against driving and parking the same commute, which popularised the annual savings figure now quoted in transit marketing. Tax policy reinforced it: the qualified transportation fringe benefit under section 132 of the Internal Revenue Code, established through the Energy Policy Act of 1992 and expanded by later transportation legislation, allowed employees to pay for transit with pre-tax income up to a monthly cap adjusted annually for inflation. The climate dimension entered mainstream reporting after the Federal Transit Administration published analysis in 2010 on public transportation and greenhouse gas emissions, which produced the per passenger mile emission factors used in modern mode comparisons.