|Fig. 1: An image of traffic congestion that could be alleviated with more efficient transportation. (Source: Wikimedia Commons)|
In the modern world, commuting has become a big point of our daily lives as we live increasingly far distances from places of work - just a look at local highways during the morning and afternoon commute will show traffic that looks like the congestion illustrated in Fig. 1. As of 2013, the average commute to work is approximately 25.8 minutes, which is almost an additional five minutes to that of the 1980s.  This additional amount of travel time may not seem too daunting, but time (and energy used) adds up quickly. Similarly, the most common mode of transportation of US commuters is driving alone, with 76.4% of Americans doing so, and another 9.4% carpooling, as seen in Fig, 2.  That leaves only ~12% of commuters walking or taking some type of public transportation.  With such little utilization of public transportation across much of the country, it is important to consider the amount of energy necessary to feed this seemingly inefficient sector of our society. Transportation alone accounts for a sizeable 28% of US energy demands and, of that, 96% of that energy comes in the form of petroleum (about 70% of the entire US's consumption of petroleum).  With these high levels of energy consumption in mind, it begs the question, but what type of transportation would be the most energy efficient?
|Fig. 2: Percentage of workers using each mode of transportation.  (Source: S. Tran)|
For the purposes of this analysis, we will look at some of the most common types of automated forms of transportation in the US. This will include transportation by private automobile, urban transit bus, commuter rail, or domestic air travel. We will be examining the efficiency of each method of transportation by referring to their passenger miles per diesel equivalent gallon (DEG), which also takes into account how many people, on average, will be transported via this method and factors it into its energy use efficiency.  Thus, the larger the number listed below in each category, the more fuel efficient it is considered.
This was calculated by first converting the total annual fuel used by each mode to units of DEG:
|Annual DEG||=||Fuel Energy Content (btu/gal)
Diesel Energy Contgent (btu/gal)
|× Annual Fuel (gal)|
Thus, giving of the DEG to calculate the energy intensity measurement mentioned above (Pass-mi/DEG):
|Passenger Miles per DEG (pass-mi/DEG)||=||Annual Passenger Miles
With this in mind, the efficiency of the highlighted methods of transportation have been broken down and calculated as follows: 
Private automobile: As mentioned above, this is the largest sector for commuting in America. This category has a high pass-mi/gal of 84.1 and a low of 25.2, resulting in an average of 27.9 pass-mi/gal when considering only lone drivers. When carpooling (two people or more) is taken into account this average increases to an average of 55.9 pass-mi/gal (high of 121.1 and low 36.3).
Urban Transit Bus: This mode of transportation only comprises 2.52% of the commuters in the U.S. , and includes the use of primarily diesel- powered, rubber-tired vehicles for fixed route scheduled service within an urban area. It has a high of 250 pass-mi/gal and a low of 3.6 pass-mi/gal, with an average of 70.5 pass-mi/gal.
Commuter Rail: Transportation that uses electric or diesel-powered locomotives that pull passenger cars for local, short- distance travel between a central city and adjacent suburbs. It has a high of 169.5 pass-mi/gal and a low of 32.1 pass-mi/gal, with an average of 90.3 pass-mi/gal.
Domestic Air Travel: This includes flights only within the U.S., and it has an average of 54.8 pass- mi/gal.
The results from this analysis indicate that the most energy efficient type of transportation is public transportation - essentially, the more people that travel via a certain mode of transportation, the more efficient it is. While this is true as deemed by the averages above, a closer look at the lower end of the pass-mi/gal calculations, demonstrates that this may not actually be the case when looking at the bigger picture, as traveling by bus can be even less efficient than by driving a car by oneself. Thus, there is not necessarily one mode of transportation that is the most energy efficient, as the amount of energy used for any type of transportation can vary greatly within that individual category. However, when looking at this data from a practical standpoint and trying to determine how to make a more informed, energy-efficient choice with respect to one's daily commute, looking into highly utilized (and thus increasingly efficient) public transit methods within one's city or organizing car pools of one's own.
© Stephanie Tran. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. McKenzie, "Who Drives to Work? Commuting by Automobile in the United States: 2013," U. S. Census Bureau, ACS-32, August 2015.
 Hidden Costs of Energy: Unpriced Consequences of Energy Production and Use (National Academies Press, 2010), pp. 154-56.
 Updated Comparison of Energy Use and CO2 Emissions From Different Transportation Modes," M. J. Bradley and Associates, April 2014.