3. TRANSPORTATION CALCULATIONS

The ecological footprint of transportation is comprised of fossil fuel land to absorb the CO2 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
m2/yr
=
m2
*
Mj
*
Liters gasoline
*
km traveled
*
 
*
1.5
Mj
m2 house/yr
km

In a sustainable world, burning gasoline requires vegetated land to absorb CO2 emissions. Wackernagel et al estimate that 10,000 m2 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 50-63% 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 G84-85) is estimated to be between 0.92-3.77 Mj per km. Train energy intensity ratio (cells G86-87) varies between 2.46-3.09 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
m2/yr
=
m2
*
Mj
*
km
*
Person hours flown
*
class multiplier
Mj
Km traveled
hour

Here, too, the carbon sequestration ratio for jet fuel is 10,000m2/71,000 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.

Built-up land

Built-up 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
m2
=
miles
*
meters
*
meters
*
1
*
miles
*
gallons (or liters)
miles
miles
gallon

The total rural and urban highway miles in the US is 3.9106 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 3.141011m2. A total of 2.51012 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.