3. TRANSPORTATION CALCULATIONS
The ecological footprint of transportation is comprised of
fossil fuel land to absorb the CO_{2} emitted from burning fuel and
the built up land for highway infrastructure.
Fossil fuel land
Fossil fuel land for automobiles (cell G90) is calculated
using the following formula:
Land area

=

Carbon sequestration ratio

*

Energy intensity ratio

*

1
gas mileage 
*

Quantity in metric or US standard

*

Metric conversion factor, if needed

*

extra embodied energy factor of car manufacture
and maintenance

m^{2}/yr

=

m^{2}

*

Mj

*

Liters gasoline

*

km traveled

*


*


Mj

m^{2} house/yr

km

In a sustainable world, burning gasoline requires vegetated land to absorb CO_{2} emissions. Wackernagel et al estimate that 10,000 m^{2} of forest can absorb the carbon emitted from the production of 71,000 Mj. The energy intensity ratio of gasoline is 35 Mj/l. Building and operating a car consumes substantially more energy than just the gasoline needed to power it. The embodied energy of automobile manufacture includes the energy needed to run manufacturing plants, refine petroleum, operate automobile dealers and parts stores, and running the construction equipment for building and maintaining highways. This embodied energy can be 5063% higher than direct fuel use (Wackernagel and Rees 1994, Herendeen 1998). In this spreadsheet, Wackernagel et al. estimate an embodied energy factor of 1.50, or 150% direct fuel use is required to build and operate a car (cell G90). Fifteen percent is additional energy to build the car, and 35 % is the indirect energy consumed to build the physical infrastructure needed for automobile use (highways, bridges, etc.).
The energy intensity ratio for using buses (cells G8485) is estimated to be between 0.923.77 Mj per km. Train energy intensity ratio (cells G8687) varies between 2.463.09 MJ per km traveled, with the lower value associated with more efficient intercity travel.
Airline travel (cell G89) uses a similar formula to that of automobile travel:
Land area

=

Carbon sequestration ratio

*

Energy intensity ratio

*

Airplane speed

*

Quantity in metric or US standard

*

Waste factor, if needed

m^{2}/yr

=

m^{2}

*

Mj

*

km

*

Person hours flown

*


Mj

Km traveled

hour

Here, too, the carbon sequestration ratio for jet fuel is 10,000m^{2}/71,000 Mj energy. Jet airplanes require 3.34 MJ per kilometer distance traveled (energy intensity ratio). This ratio also includes extra embodied energy factor of airport infrastructure. A typical flight speed is 800 = Km/hr. The class multiplier adjusts the footprint based on whether the flight is economy or business class. A multiplier of 0.95 is used for economy class and 1.1 for business class, reflecting the fact that it is less efficient to fly fewer business class passengers than economy class passengers. Person hours indicates the number of hours flown per person.
Builtup land
Builtup land required for automobile use can be calculated
using the following formula and data.
Land area

=

Total highway length

*

Conversion factor

*

Highway width

*

1/Vehicle miles traveled

*

Vehicle gas mileage

*

Personal gas consumption

m^{2}

=

miles

*

meters

*

meters

*

1

*


*


miles

miles

gallon

The total rural and urban highway miles in the US is 3.9´10^{6} miles (Bureau of Trans. Stats.). There are 1609 meters/mile. The estimated average width of highways is 50 meters. Multiplying these three quantities yields a total highway area of 3.14´10^{11}m^{2}. A total of 2.5´10^{12} vehicle miles are traveled per year in the US (Bureau of Trans. Stats.). An average car gets 20 miles/gallon, and there are about 3.78 liters per gallon. Note how all of the units cancel when multiplying through the right hand side of the equation. The personal footprint contribution for built up area required for taxis and buses is assumed to be 5% and 2%, respectively, of the total distance traveled using these modes of transportation.