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Energy intensity: driving versus flying

Hydrocarbon Engineering,


Introduction

This report analysers recent trends in the amount of energy needed to transport a person in the US a given distance either in a light duty vehicle or on a scheduled airline flight. After observing that the energy intensity of driving is greater than that of flying, calculations are made to estimate how much improvement would need to be achieved in either vehicle fuel economy or passenger load to make driving the less energy intensive of these two modes of transportation.

Recent trends

The variable of interest was BTU per person mile. For flying, ‘person mile’ refers to passenger mile, while for driving it refers to occupant mile. For flying, domestic operations of all certified air carriers were considered. For driving, all light duty vehicles were included.

Research data indicates that in 1970 the energy intensity of driving was about half that of flying. However, the advantage of driving decreased with each five year increment examined. Indeed, the situation revered in 2000, and the advantage of flying increased from there on. For 2010, the latest year analysed, the energy intensity of driving was 57% greater than that of flying. Over the course of the 40 years examined, the energy intensities of both driving and flying decreased. However, the improvement for driving (17%) was substantially less than for flying (74%).

How to improve the energy intensity of driving

The energy intensity of driving depends on two primary variables: vehicle fuel economy and vehicles load. As vehicle load increases, the amount of fuel consumed per person mile decreases. The fuel economy of the US fleet of all light duty vehicles improved from 13.0 mpg in 1970 to 21.5 mpg in 2010. However, during the same period vehicle load decreased from 1.90 persons to 1.38 persons.

One way for driving to be less energy intensive than flying is to improve vehicle fuel economy by more than the current ration of the energy intensities of driving and flying. That ratio is 1.57. Consequently, at the current vehicle load, vehicle fuel economy would have to be at least 33.8 mpg.

Increasing vehicle load

The calculations in this section provide an estimate of the needed vehicle load to yield an equal energy intensity for driving and flying. The following assumptions were made in these calculations:

  • Average vehicle curb weight with fuel: 3500 lbs.
  • Average weight of a person with luggage: 250 lbs.
  • Vehicle fuel economy: a linear function of total vehicle weight.

The results indicate that a vehicle load of 2.3 persons would yield an energy intensity of driving that is the same as the current energy intensity of flying. Therefore, without any improvement in vehicle fuel economy, an average vehicle load of more than 2.3 persons would result in the energy intensity of driving to be less than that of flying.

Discussion

Improvements needed

Necessary improvements in vehicle fuel economy or vehicle load for the energy intensity of driving to match the current energy intensity of flying:

  • Vehicle fuel economy: 57%, from 21.5 mpg in 2010 to 33.8 mpg.
  • Vehicle load: 67%, from 1.38 persons in 2010 to 2.3 persons.

It would not be east to achieve either of the two changes outlined about. Let us first consider vehicle fuel economy. Although the fuel economy of new vehicles is continuously improving, and these improvements are likely to accelerate given the new corporate average fuel economy standards, changes in fuel economy of new vehicles take a long time to substantially influence the fuel economy of the entire fleet. For example, the 14.5 million light duty vehicles sold in 2023, accounted for only approximately 6% of the entire fleet of light duty vehicles.

A historical perspective illustrates the daunting task. The two points above indicate that an improvement of at least 57% in vehicle fuel economy of the entire fleet of light duty vehicles would be required. In comparison, during the 40 years that were examined in this study, vehicle fuel economy improved by only 65%.

The required increase in vehicle load of at least 67% might be even more difficult to achieve. This is the case because vehicle load has recently been continuously dropping, from 1.90 in 1970 to 1.38 in 2010.

It is important to recognise that the energy intensity of flying will continue to improve. Indeed, from 1970 to 2010, the energy intensity of flying decreased by a larger percentage than that of driving (74% versus 17%). Consequently, because the future energy intensity of flying will be better than it currently is, the improvements outlined above underestimate the improvements that need to be achieved for driving to be less energy intensive than flying.

Electric vehicles

The presented energy intensities of driving slightly underestimate the actual intensities because the electric energy consumed by plug in hybrid electric vehicles or fully electric vehicles was not included. However, such vehicles currently represent less than 1% of all vehicles on the road.

Driving trips versus flying trips

The average length of a driving trip is currently approximately 9 miles. On the other hand, the average domestic flying trip is currently approximately 100 times longer. Thus, driving and flying serve different general purposes, with driving used mostly for trips that are too short for flying. However, long distance driving represents a subgroup of driving trips for which flying is a viable alternative.

As the trip length increases, so does the average fuel economy of driving. This is the case because long distance driving is frequently done on limited access highways where vehicle fuel economy is better than the average fuel economy over all roads that were included in this analysis. Similarly, as the trip length increases, so does the average fuel economy of flying. This is the case because airplanes use a disproportionate amount of fuel during takeoffs. For example, one estimate is that on short trips, takeoffs are responsible for as much as 25% of the total fuel consumed. 

Adapted from Michael Sivak's report by Claira Lloyd.

Read the article online at: https://www.hydrocarbonengineering.com/gas-processing/10012014/driving_versus_flying34/


 

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