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Jan 25, 2013

Driving Miss Hazy: Will driverless cars decrease fossil fuel consumption?


There was a time when manual transmissions always outperformed automatics on the track, but you won’t find a clutch on today’s F1 supercars anymore. Those have been replaced by semi-automatic paddle shifters mounted to the steering column. Even on the street-legal consumer side, acceleration is a shifting landscape. While six-speed manuals such as the Lingenfelter Chevy Corvette and Dodge Viper Hennessey Venom still hold the top 0 to 60 mph times, today’s automatics are increasingly accelerating faster and getting better fuel economy than their manual counterparts in what amounts to a significant changing of the guard.

In the automotive world—as in computing, Jeopardy!, and sometimes even chess—technology is outperforming humans.

With much talk about driverless car technology lately, that begs a number of questions. One example: Vehicle fuel efficiency, hybrids, and electric vehicles are central to a transforming, cleaner, greener transportation system; What will driverless cars mean for fossil fuel consumption in the transportation sector? At RMI, we’re very interested in the answers to this question.

Individual driverless cars come with important opportunities for increased efficiency. Groups of driverless cars likewise have the potential for even greater collective efficiencies, for example through shared reduced drag. And driverless vehicles seamlessly connected to smart infrastructure offer even greater promise still.

Consider hypermiling, which focuses on maximizing fuel efficiency through driving technique and has a cult of devout enthusiasts pushing the limits of their vehicles’ mpg. For example, one driver on hypermiling site ecomodder.com is coaxing more than 86 mpg out of a 1988 Honda Civic, besting its EPA rating by close to 175 percent.

Hypermiling requires a whole new approach to driving—how fast you drive; how you accelerate, brake, and approach hills and traffic lights. Most drivers aren’t well versed in how to hypermile; they just try to maintain (or not overly exceed) the speed limit. But driverless cars could do the hypermiling for us. (The Honda Civic Hybrid’s Econ Button is already a small step in that direction, prioritizing efficiency over a driver’s lead foot tendencies.)

And while hypermiling is often derided as “a fun way to drive slow,” driverless cars—via connected autonomy—could actually enable us to get places faster while still maintaining hypermile-like fuel economy.

There’s a memorable scene in Days of Thunder when a young Tom Cruise teaches Nicole Kidman the concept of drafting: “When one car tucks in behind another, two cars go faster than one. They divide the air resistance between them. Now here’s where it gets interesting. The lead car has to floor it to hit 200 mph, but the car that’s tucked in behind doesn’t. It can go just as fast and still have power in reserve.”

In the context of driverless cars, drafting becomes platooning, a grouped “train” of autonomous, connected cars following a lead car. The SARTRE project—fully operational today and potentially ready for consumer deployment within 10 years—is the latest iteration of a concept that first gained popularity in the 1990s. By traveling within 13 feet of one another, the platooned vehicles reap great efficiency gains. A 1995 PATH (Partners for Advanced Transportation TecHnology) study showed fuel use reductions up to 20 percent at this distance; even greater reductions are possible with larger platoons and/or tighter spacing between vehicles.

Now here’s where driverless cars, hypermiling, and speed all potentially intersect. The National Highway Maximum Speed Law of 1974, motivated by the 1970s oil crisis, gave us the 55 mph speed limit, targeted at keeping U.S. drivers at peak fuel efficiency. In 1995, the law was abolished and speed limits now vary widely state by state, though 55 mph remains common on many of the nation’s highways. But what if driverless cars—by being able to safely maintain much closer following distances in platoons than human drivers—could actually increase the speed of peak efficiency? We’d get places faster while still using less fuel.

The opportunities bound up in driverless cars are many. They’re highly compatible with car-sharing programs (more on that in a future post). They have implications for public transportation systems (ditto). If you’re feeling drowsy on a long-haul drive or at the end of a tough day at work, a driverless car could let you sit back and relax, or even take the wheel if it senses you’re driving erratically; ditto if you find you’ve unexpectedly had too much to drink—rather than call a taxi, a driverless car could take you home. By feeding information into (and obtaining info from) a shared database akin to the smartphone app Pothole Alert, they could avoid road hazards or even notify the local highway department or department of public works to road repair and maintenance needs.

But let’s return to the fuel consumption issue. Driverless cars connected to the traffic light system could communicate with an upcoming light and have it stay green or change to green, so that you could maintain cruising speed and optimal fuel efficiency (not to mention avoiding an unnecessary delay at the light). And of course there’s the familiar issue of parking, especially in urban environments where it accounts for 30 percent of all traffic. Autonomous cars and a smart parking system could allocate parking spaces to the nearest vehicles in need of a spot, and vehicles could navigate to those spots directly and quickly, reducing the fuel consumption, emissions, and time associated with searching for a spot.

There are pitfalls to be wary of, however. One giant elephant in the driverless car room is vehicle miles traveled (VMT). It’s tempting to believe that driverless cars will reduce VMTs through various efficiencies, but the opposite could also happen. In a highly autonomous vehicle future, we may do far more with our cars than we currently can—think of Pierce Brosnan’s James Bond calling his BMW 750 via cell phone in Tomorrow Never Dies, or David Hasselhoff’s Michael Knight and his autonomous car, KITT, each chasing down different suspects in Knight Rider.

What if our driverless cars of the future have a valet mode that allows them to park themselves, becoming not just driverless but also passengerless? What if they run errands for us, such as picking up lunch or dry cleaning or the kids at soccer practice? What if driverless cars—and the efficiencies and flexibility they offer—actually incentivize us to drive more? This “rebound effect” could increase VMT and fuel consumption, eating into the potential energy savings we’d otherwise expect from highly efficient autonomous vehicles. We must be prepared to address these undesirable yet entirely possible outcomes.

Robert Frost wrote, “Two roads diverged in a wood, and I— / I took the one less traveled by, / And that has made all the difference.” With the dawn of driverless vehicle technology, we are at a fork in the road, each path with its own consequences, positive and negative. Driverless vehicle technology can and likely will make a difference. But what difference will it make? It has the potential to be radically more efficient than the status quo, but it also might increase—possibly substantially—VMTs and fuel consumption. So how do we ensure that our future state is more of the former, and less the latter? We’ll explore that in coming posts.

Recommended Reading

Images 1 & 3 courtesy of SARTRE.

Image 2 courtesy of PATH.

Join the Discussion

Showing 1-10 of 12 comments

January 25, 2013

What about the eventual laws that will outlaw driver vehicle control - will they force everyone to operate autonomous vehicles? theres going to be a very interesting period of time where you have these efficient autonomous vehicles and the crazy guys in BMWs who pass everyone at every turn.

January 29, 2013

Hi Madden. That's a big question, and one for the legislative crowd to tackle. Rather than focus on that topic, we wanted to use our expertise to analyze what it could mean for the energy spectrum (both positive and negative). We see it as an important part of the conversation, and plan to dive further into the topic as the series continues.

But I'll play: I've been trying to think of your question from a late 1800s perspective, when really good horse-n-buggy drivers said to themselves: "Will they force us to drive these confounded machines?!" Growing up in Pennsylvania, I can assure you that you can still drive a buggy around if you want to, which I do not. I see this going the same way. Most will let the robot drive. Jay Leno will still take out his classics by hand, and you'll see the die-hard drivers on tracks and off-road...which is exactly the same place horse-riding happens out these days.

If you ask me, driving off-road and on tracks is more fun anyway. http://www.youtube.com/watch?v=6VyXpPspxlA

January 31, 2013

Another possibility though is that driverless technology is the enabling technology that completely changes how we view and use the automobile. Rather than owning a car it could be turned into a service and flow business where people contract for transportation. We'd use cell phones to order a vehicle and it would come to us. Then when we arrived at our destination it would go off and take other passengers. Since vehicle utilization is so low currently it opens the possibility of giving everyone better transportation with fewer cars, and fewer parking lots. Users could contract based on size of vehicle and the number of miles traveled each month.

If the driverless car is also electrified it can sense when it will need to be charged and go off and charge itself.

January 31, 2013

Assuming this all actually works out and the need for perfection in safety systems is satisfied (akin to the challenge of nuclear power plants) it may have great benefits for fuel efficiency. But what about the equally pressing issue of resource consumption? We here in the US (and the more developed nations in general) are largely shielded from the reality of resource extraction, but it is exacting a huge toll in the poorer (ironically - considering the resource wealth at their feet) and less powerful regions of the world, where billions living at what we would consider poverty level are faced with water pollution, soil pollution, and loss of arable land that in some cases threatens their survival in order to feed our demand for every adult to own a two-ton contraption in order to carry our bodies from place to place. Public (or shared quasi-pubic) transportation seems to be essential to address this issue of resource extraction.
Another factor to consider is how we can reduce both the need for speed and the need for distance. If more destinations were within easy walking, biking, or modest transit range, it would greatly reduce both energy and other resource consumption and have indirect benefits of improved health and more social connectivity. In the words of Dr. Richard Jackson, "our pattern of land use is a public health disaster”. And why do we need to do everything so fast? This is an historically recent social and economic phenomenon, not a natural state of the human species. The capacity to make the journey itself a meaningful part of our lives is lost in the maze of pavement and isolation inside mechanized objects that unnaturally constrain the natural human interaction cues of eye contact, body language, and verbal exchange. Technical fixes to improve efficiency are a necessary part of making a transition to a sustainable future, but until we learn how to restore some deeper health and community intactness to our lives as human beings on this extraordinary planet, all the technical fixes in the world won't bring us happiness or a way of life that is truly sustainable.

January 31, 2013

The major effect of driverless cars would be to make traffic accidents rare, to the point that a motorcycle is almost as safe as an SUV. Urban driving makes up 65% of VMT; a motorcycle can have an order of magnitude less mass than a car (average US weight is 4000 lb) and move people just as well on less energy. See my paper on http://www.qi2.com/index.php/transportation
Cooperative Adaptive Cruise Control (CACC) allows a 0.6 sec gap between cars. This is 3 lengths at freeway speed, where the PATH data you cite show that there is little effect on fuel savings. Safety and real fuel savings require that the platooned vehicles mechanically couple.

January 31, 2013

The lead car burns more fuel, but all the following cars benefit. This is similar to riding a bicycle in a pace line: The lead rider burns out quickly, and pulls to the left (away from the curb/edge) and drops to the back of the pace line. In this way the entire pace line is able to ride at a faster pace. A similar routine needs to be included in the software for the autonomous vehicle control so all of the vehicles share in the benefit. This would require all the vehicles to be able to communicate with each other, which they need to do for many other reasons anyway.

In the photo at the top of this article, there are two large trucks followed by several cars in a "train." The pace line software would need to account for relative size (frontal area) and put the larger vehicles in their own group at the front of the rotating lead, with the smaller vehicles arranged by decreasing size following.

In bicycle pace lines a congenial group will allow the older rider/s to stay at the back and not take a turn at the lead. They will wait for the retiring lead as he moves back next to the pace line, and move to the left when he is close to draft him, slow with him as he moves into place in the line, and follow.

In bicycle pace lines, a very small rider does not create enough advantage that a larger rider can follow him. A tandem bicycle will be faster in a descent down a long grade, so others want to get "on his wheel." But if the first one to do so is really small, none of the others in the group will be able to keep in the pace line, and so will form a slower group, and follow.

January 31, 2013

This is a solution waiting for a problem.

February 1, 2013

To work towards the world in which we all wish to live, Hypermiling should be as integral in Driver Training as is Defensive Driving (until autonomous vehicles achieve mainstream adoption). The slower speeds would increase safety as a by-product. I predict that the notion of making 'driving slow fun' will be a major factor in fostering mainstream adoption of these more eco-friendly driving practices. Similarly, we may even see a cultural shift away from 300-400 horsepower family sedans in the next few years as people realize they need significantly less power for their daily lives and can save money by making the switch.

As an (old school) automotive enthusiast, I fear a forced transition to autonomous vehicles, but the potential benefits of safety & decreased travel times coupled with improved fuel economy are hard to ignore.

In my dream world, the government would issue Porsche 918's to all citizens...that'd best of both worlds! http://blog.rmi.org/blog_the_porsche_918_spyder_phev

Well written piece and looking forward to the article on Car Sharing!

February 3, 2013

Another thing to consider is efficient use of highways. Smart cars can avoid many of the causes of traffic jams, not only by increasing the effective capacity of a given road through more density, but also reducing incidence of traffic accidents, or even "rubbernecking" and slowdowns due to sun glare, etc. Taken one step further, excess traffic could be reduced by smart rerouting and better trip scheduling. The technology for much of this is already present in smartphones, gps, alerting systems and so on.

But many of the benefits of smart and driverless cars only work when adoption rates are sufficiently high. One car drafting another today is probably illegal in most states -- also known as tailgating! I agree with another comment regarding the need for legislative support.

February 6, 2013

Great comments, all. We really appreciate them and keep 'em comin!

For those bringing up the whole-systems implications of increased highway efficiency and vehicle ownership models, we will be exploring this in our next blog.

Tyler, great comments and outstanding article! I recommend everyone read that one. As far as your platooning comment, I'd like to point out that Volvo has had success with a 4 meter gap at 55mph. On their XC90, this is equivalent to 0.83 car lengths at highway speeds. Based on PATH's findings that could mean a 16% reduction.


Brian, I strongly agree that we all deserve the new Porsche! A lesson or at least reading the ecomodder recommendations would be of great benefit to society and the individual wallet. Even a simple MPG gauge common in RVs could at least help people begin to understand exactly what driving decisions are costing them the most money. If TPMS is a federal requirement, so should be MPG gauges!

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