Autonomous Vehicles

Will Vehicle Energy Use Increase or Decrease with Automation?

There is an ongoing debate within the transportation analysis community over whether automated vehicles will reduce energy use and greenhouse gas emissions, or increase them. On the one hand, automated vehicles can both drive more efficiently and be designed to be lighter, saving energy. However, the convenience of not having to pay attention on the road may lead people to use them more, increasing energy consumption. Which effect will tend to prevail?

An interesting new paper by Wadud et al. (2016) does an excellent job of trying to answer this question. They quantify the range of energy impacts of automated vehicles across 12 different effects, including eco-driving, platooning, and right-sizing. The work builds upon previous research in this area by Brown et al. (2014), Morrow et al. (2014), and a few others.

One of the most innovative things Wadud et al. do is provide a novel approach for estimating the increase in travel demand using a travel cost elasticity relationship. That is, they postulate that the change in distance a vehicle will drive annually depends on the ratio of total cost of vehicle ownership before and after automation, raised to an exponent called the elasticity.

While speculative, it offers a defensible way to estimate the increase in travel usage if the costs of insurance, fuel, and importantly, people’s time, decrease with automation. I applaud them for attempting to put more quantitative bounds on the potential of automation to increase or decrease transportation energy use.

To examine overall effects, they develop four scenarios that emphasize different groups of features, and conclude that future energy use may range from about a 40 percent decrease to a 100 percent increase relative to today. Both the studies by Brown et al. and Morrow et al. reached many of these same conclusions, though their estimated range of changes were higher.

How to reduce energy use with automation

However, I think that the study misses some important synergies that could further reduce energy use beyond what is captured in their analysis. In particular, they only estimate the effect of shared mobility on vehicle usage (vehicle kilometers traveled or VKT), without considering its potential to enable greater use of electric vehicles, which could result in large energy savings.

While they discuss the potential for automation to enable greater use of alternative fuels (including electricity), they stop short of making any quantitative estimates in this regard.

This is a significant shortcoming. In my paper published last year in Nature Climate Change, I focused on this effect, and concluded it has the potential to be strong because it is coupled with lower total operating costs, driving potentially large adoption.

Smaller (one- and two-seat) vehicles could further reduce costs, making shared electric vehicles even more appealing. Because electric motors are several times more efficient than gasoline-powered engines, it could result in significantly lower energy use per VKT.

Electricity already has lower greenhouse gas emissions per unit energy compared to petroleum, and potential policy changes encouraging more renewables and less reliance on fossil fuels, such as recent California legislation or the federal Clean Power Plan, could further reduce emissions.

Therefore, shared electric vehicles represent a very important way to lower greenhouse gas emissions from the vehicle transportation sector in a way that simply reducing petroleum use cannot. My study concluded that these vehicles combined with a greener grid could still result in lower greenhouse gas emissions even if travel demand due to vehicle automation were higher.

Do you think that automated vehicles will increase or decrease average energy use? How about greenhouse gas emissions? Share your thoughts in the comments section below.

Please note that this article expresses the opinions of the author and does not reflect the views of Move Forward.


  • Tyler Folsom
    16. March 2016 at 18:57

    Automation offers the path to greatly reduce greenhouse gasses by making most urban transportation run on renewable energy. Let’s not forget that we are not at the mercy of the market. Automation has the side effect of enabling 100% enforcement of traffic rules, including vehicle minimum fuel efficiency and maximum weight. One regulation might be a limit on daily travel distance under automation. This limit could be used to discourage sprawl. A variant is to heavily tax excessive travel.

  • Jeffery Greenblatt
    19. March 2016 at 17:05

    Interesting suggestion, Tyler! Daily or annual travel distance (commonly known as vehicle miles or kilometers traveled, VMT or VKT) could indeed be limited through a combination of technology to track distance, and legislation to impose standards. However, let’s be careful not to focus on VMT/VKT for its own sake, as energy and greenhouse gas emissions are the real “bads” to combat.

    The use of a high efficiency vehicle, or better still, an electric vehicle with much lower greenhouse gas emissions per VMT/VKT than a petroleum-powered vehicle, could be unnecessarily limited even though its environmental footprint is much lower than a conventional vehicle’s. And a shared vehicle driven many times farther than a privately owned vehicle could similarly be punished even though it would service many people’s needs and perhaps eliminate car ownership among those users.

    Sprawl is bad in that it burdens roadways and historically generated more air pollution, but this relationship may be changing with new vehicle technology. And self-driving cars may make roads safer and more pleasant to be on, so the burden of a longer commute is easier to endure.

    So in principle you’re right, but one has to be careful that new policies don’t create unintended consequences.


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