Calculating the carbon footprint of a bus requires a few basic lines of thought. For the most accurate results, you can keep track of all the gasoline used to fuel the bus. Alternatively, you can estimate the footprint based on the measured fuel economy of a particular bus, or averages for buses in general. Though the resulting numbers may sound high, keep in mind that they are almost invariably lower than the footprint of private transportation if all the bus riders were operating their own vehicles
Calculate Based on Fuel Consumption
Keep careful track of how many gallons of gasoline the bus uses over a period of several days. Multiply the gallons used by 8.8, which is the number of kilograms of carbon dioxide released by the combustion of a gallon of gas (see References 1). Divide the result by the number of days during which you kept track of fuel consumption. The answer is the bus's average daily carbon footprint. The longer you keep track, the closer you come to a true average.
Calculate From Bus Mileage
Determine the bus's typical mileage. You can use either the listed value for that model of bus or measure it yourself for greater accuracy. Be careful to distinguish between mileage in the city, which is often lower, and mileage for noncity driving. The easiest way to measure mileage is to write down the mileage on the odometer when the tank is filled and then again when it's near empty. Divide the number of miles in a particular trip by the mileage to determine the gallons of gas used. Multiply this number by 8.8 to find out the bus's carbon emissions for a trip of that length.
Calculate Based on Passenger Miles
According to the World Resource Institute, city buses emit an average of 0.66 kg of CO2 per passenger mile, while long-distance buses making trips longer than 20 miles emit 0.18 kg of CO2 per passenger mile (see References 1). Determine which type of bus is being used and multiply the miles one passenger travels to calculate the carbon footprint of that particular passenger's trip. To determine the footprint of the whole bus, multiply the appropriate coefficient, either 0.18 or 0.66, by the average number of passengers per day, then multiply by the average number of miles each passenger travels. These numbers are based on a typical number of passengers; operating an empty bus does not translate into zero carbon emissions.
Truly measuring the carbon footprint of a bus means considering the embodied energy, or the energy used to produce the bus. Based on the energy it takes to manufacture metal, glass, plastic and other automobile parts, about 100 gigajoules of energy are used in producing a car. Each gigajoule corresponds to roughly 98 kg of carbon dioxide, for a total of 9.8 metric tons per car. Buses weigh between 10 and 15 tons, so the embodied energy of a bus is closer to 100 or 150 metric tons of carbon. Buses operate in constant service for 10 or 20 years, so their lifetime mileage is often much higher than a car's and makes up for the greater initial footprint. (See References 2.)
Eric Moll began writing professionally in 2006. He wrote an opinion column for the "Arizona Daily Wildcat" and worked as an editor for "Persona Literary Magazine." He has a Bachelor of Science in environmental science and creative writing from the University of Arizona.
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