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Making driving less energy-intensive than flying

Energy intensities of flying (blue) and driving (green), 1970-2010. Data: Sivak, UMTRI-2014-2. Click to enlarge.

Currently, the energy intensity (BTU per person mile) of driving is 57% greater than that of flying, according to a new analysis by Dr. Michael Sivak, Director, Sustainable Worldwide Transportation at the University of Michigan Transportation Research Institute (UMTRI). To make driving less energy intensive than flying, the fuel economy of the entire US fleet of light-duty vehicles would have to improve from the current 21.5 mpg (10.9 l/100 km) to at least 33.8 mpg (7.0 l/100 km) at the current vehicle load, or vehicle load would have to increase from the current 1.38 persons to at least 2.3 persons.

In the report, Sivak considered domestic operations of all certified air carriers were considered and all light-duty vehicles (cars, SUVs, pickups, and vans) over the past 40 years. During that period, the energy intensities of both driving and flying decreased. However, the improvement for driving (17%) was substantially less than for flying (74%).

In 1970, the energy intensity of driving was about half that of flying, but decreased with each five-year increment examined. The situation reversed in 2000, and the advantage of flying increased from then on. For the latest year analyzed (2010), the energy intensity of driving was 57% greater than that of flying.

In 2010, driving was the least efficient of five major transportation modes (4,218 BTU/person mile), with rail (Amtrak) being the most at 1,668 BTU/person mile. Motorcycles were the second most efficient (2,675 BTU/peson mile), followed closely by flying (2,691 BTU/person mile). Transit buses came in fourth (3,347 BTU/person mile).

Sivak then used the ratio of energy intensities of driving and flying (1.57 (4,218/2,691 = 1.57) to calculate, at the current vehicle load, the requisite fuel economy to bring driving to parity.

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.

It would not be easy to achieve either of the two changes … 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. This is the case because it takes a long time to turn over the fleet. For example, the 14.5 million light-duty vehicles sold in 2012, accounted for only about 6% of the entire fleet of light-duty vehicles.

—Sivak, UMTRI-2014-2

Simply looking at the history provides perspective on “the daunting task”, Sivak notes. Although a minimum 57% improvement in vehicle fuel economy is required for parity between the two modes, during the 40 years that were examined in the study, vehicle fuel economy improved by only 65%.

Reducing vehicle load by at least 67% might be even more difficult to achieve, Sivak concludes; vehicle load has recently been continuously dropping, from 1.90 in 1970 to 1.38 in 2010.

However, Sivak concludes, although flying is less energy intensive than driving, flying is a viable alternative to driving only for a subset of driving trips that involve relatively long distances.


  • Michael Sivak (2014) “Making Driving less Energy Intensive than Flying” (UMTRI-2014-2)


Bob Wallace

"To make driving less energy intensive than flying, the fuel economy of the entire US fleet of light-duty vehicles would have to improve from the current 21.5 mpg (10.9 l/100 km) to at least 33.8 mpg (7.0 l/100 km) at the current vehicle load, or vehicle load would have to increase from the current 1.38 persons to at least 2.3 persons."

1) Pass a law requiring all light vehicles sold starting in 2018 be PHEVs or EVs.

2) Create a "Crush the Clunkers" program that sets a sweet trade in value for low MPG vehicles.

One decade. Problem solved.


Good idea BW.

I more than meet the objective with a Camry Hybrid.

France tried something very similar, since no equivalent offset was found to cover the high cost of 'Crushing the Clunkers' the program may come to an end.

USA with its current very low Fed fuel tax could do better with a progressive increase of the liquid fuel tax to offset the cost of such program?

Bob Wallace

We could calculate the health savings resulting from a 80%, 90% reduction in fuel use and use that money.

If we really wanted to be creative we could the diverted funds from Oil War IV.

The real issue here is avoiding the cost of extreme climate change. We're going to spend small money soon or massive money down the road.


Interesting and not very surprising. But doesn't this merely point out the difference between competitive commercial (profit motive) and non-competitive personal (many other motivations depending on the individual)activities? There are tremendous pressures to take fuel costs out of the bottom line of a commercial airline.

In choosing personal transportation (at least in the USA) fuel economy is just one factor out of many that go into a purchase decision, if it is considered at all. We now live in world where there is a growing choice of cars that get > 33.8 mpg and which are available at all market levels. We would all be driving these except for the fact that there are alternatives available which are cheaper, or more luxurious or more satisfying to drive at each and every price point.

And diesel and hybrid technologies still exact a significant premium over conventional models, the cost of which may never realistically pay for itself in terms of fuel saved. Prius comes closest to making the argument for Hybrid technology but the line still suffers if you are allowed to compare it with conventional cars of the same size class, overall finish, and performance (other than mpg). And now that conventional cars are improving with this respect the cost premium is becoming even less justifiable. Mind you, I am glad hybrids and clean diesels exist and sure I wish the green curve was heading downward, but the fact is that the curve represents the decisions of millions of people armed with more knowledge and choices they've ever had before. Get the economy in shape and there is no need to crush the clunkers, but then you just get more inefficient cars in such an environment. No thanks if crushing clunkers means owners of older SUVs get to take advantage of the program and I'm stuck with my '99 Honda Civic. Pass a law electrifying all cars and now it becomes a world where owners of rare earth mines rule. Or make cars very expensive to own and operate through tax disincentives and risk killing an industry which is nearly happening to all car producers in Europe except for those doing business in the USA. There are no easy answers assuming there is a problem.


The problems are many. Many more are getting sick and die from man made pollution. Direct and indirect health care cost are going up every month and will soon reach 20% of GDP in many places.

The total cost (in human lives and $$$$) of increased pollution in the last 40 years is higher than the last 10 wars added together. If not checked, the next 40 years may be 3 to 4 times more costly.

What is happening in Asia's large cities (and in many other places) is the beginning of much worst days to come.


Surely the US already has a plan to get to 54 mpg by 2025.

Just implement that plan and you are home and dry.
@Bob, I wouldn't mandate how to get to 54 mpg, I would just mandate that you have to do it.

What is happening in Asia's largest cities happened in the West over about 150 years, they are just getting it all in about 20 years. The main thing is to stop people burning coal near cities.

The Chinese know this, but they are mad for growth at any price. However, as the senior people's children die from pollution related diseses, they will take action.
Nuclear, wind, solar, gas.
And get people out of their cars and onto urban rail and bus systems, just like in large western cities.


USA, Canada and Australia are still the top per capita polluters of the world at 20 to 24 tonnes/year for each of us.

China, India, Brazil and Europe are still very far behind.

Transferring factories to Asia helped to reduce our pollution growth rate but quickly increased theirs.

China is building 100,000+ Km of high speed electric rails, hundreds e-subway and urban trains, dozens of NPP, electric city buses, electric taxis, electric vehicles and bikes but it is not being done fast enough to match their industrial growth. It may take another 20 years or so before things get better. Singapore has done it and other Asian cities will do it.


I'm unsure what this paper is comparing. My prior belief is that coaches have a better fuel efficiency (well loaded) than planes partly based upon conclusions by George Monbiot. So I'm prejudiced to believe that Sivak is comparing buses on an urban cycle with planes on inter-city trips - which would be barmy. I can't find the source article to check.


All this effort to keep people in their cars? Let me point out this article flat out said travel by rail was already the most efficient so why don't we just encourage more people to use rail?

Roger Pham

Compare efficiencies of flying airliner and personal car is like comparing apples to oranges. Totally different things. It would be simpler just trying to make airliners as efficient as possible, as well as making automobiles as efficient as possible, without any comparison of the two.


One way of "making automobiles as efficient as possible" is to operate them like trains.

TEV is an E-guideway concept: EVs can travel on TEV’s electric highways at much higher speeds and in far greater safety than even the latest and greatest in high-tech cars can on a motorway today. Automatic control enables close-coupling of vehicles (described by some as ‘convoying’ or ‘platoons’), so TEV has a vastly greater passenger carrying capacity than traditional roads and even high speed commuter trains. Since the vehicles are powered directly with electricity as they travel, there is no limit to how far they can drive without recharging; in fact, batteries are recharged during the journey, during which time the EVs burn no petroleum and produce no local emissions.

Platooning improves aerodynamic efficiency, directly powering vehicles instead of charging/discharging batteries improves electrical efficiency, and the covered roadbed reduces rolling friction.

Bob Wallace

My statement is that we could get to a very low oil use point very quickly by allowing only EVs and PHEVs to be marketed. We could be close to 100 MPG before 2020 rather than 54.5 by 2025.

While building HSR is a great idea and I really wish we had some, we simply won't build it that fast. HSR would largely replace moderate length air travel but it probably wouldn't do all that much to get cars off the road. US drivers take long trips (>100 miles) much less than most people assume.

Roger Pham

@ai vin,
Thanks for the link. Indeed, TEV is a fascinating concept than can solve both the traffic jams and the range issue of BEV. This type of concept was started by Alden Self-Transit System Corp of Massachusette in the 60's, and even endorsed by then governor Volpe. Now, over half a century away, we have better technology to realize the concept. However, given the tremendous scope of the concept, the capital investment required would be monumental, as well as socio-political hurdles. Recall why and how the non-polluting electric trolley cars of LA was torn down to make ways for polluting diesel city buses, and this, in LA, where smog is a bigger problem than anywhere else.


An ultra light weight (under 1000 lbs) aerodynamic EV with large dia. thin tires, covered with very high efficiency flexible solar panels and 5-5-5 batteries could transport 2 to 4 (normal weight) passengers 500 to 800 Km/day at very low cost passenger/Km.

The same EV could transport (1 to 4) people to work and back with excess free energy, depending on distance between residence and work place.

We all worry too much about the cost of future clean e-energy.

Thomas Pedersen

Someone needs to figure out how to make the VW XL1 a lot cheaper. Cut some corners to cut cost.

A hybrid drive chain seems to be too expensive for such a vehicle. A small ICE with a glorified starter motor for boost should do it. Or even as a BEV is should achieve sufficient range simply by swapping the ICE and gearbox for more batteries (I haven't made any calculations - just brainstorming here).

The very aerodynamic body only makes sense for prolonged travel at highway speeds. To that end, it is most likely more optimal to go for an efficient ICE (diesel) than BEV. Of course, any aerodynamic car would suffer from sluggish acceleration from the meager ICE sufficient to propel it at 100 mph. An SUV has the inverse problem - even SUVs that are very quick off the line achieve less than admirable top speeds.

Anyway, like Harvey says, if we really need to, we can double the average fuel economy without breaking a sweat. And we can go beyond that as well. At the expense of some luxury, I am afraid... ;-/

Thomas Pedersen

I forgot to state the obvious about the XL1; loose the insanely expensive CFRP body. At least until production costs come down.


Plain dead (over) weight is more of a problem for the average family vehicle. Driving around with 4000+ lbs consumes too much energy unless you normally carry 10+ passengers.

Future e-cars will have to be much lighter if we realy want to reduce the energy required per passenger/Km or miles.

Ultra light weight: bodies, 5-5-5 batteries, e-motors, e-controls, e-ancillaries, windows, flexible solar cells, wheels, tires and other on-board equipment will have to be used.

The current 4000 lbs vehicles could be reduced to 2000 lbs, than to 1500 lbs and eventually to 1000 lbs for smaller units.


One ultra light weight ELIO electric 3-wheeler @ under $10K per American family could do a lot to lower the energy foot print and emission per passenger mile.

Larger families could use 2+ ELIOs.

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