Tuesday, 6 May 2014

Will autonomous vehicles save fuel?

Photo by realSMILEY via Flickr Creative Commons
Cars that drive themselves have been in the news a bit lately, with Google’s test vehicles successfully navigating in Mountain View city as well as on the highway [1]. Some enthusiasts claim that autonomous vehicles will bring better fuel efficiency and lower emissions as well as fewer accidents.  Savings will come from smoother acceleration, reduced congestion bringing less stop/start traffic jams and reduced aerodynamic drag when cars follow each other very close together (‘platooning’). Is this plausible?


Vehicle autonomy is not an all or nothing affair. There are levels of automation with increasing levels of convenience: from anti-lock brakes, through cruise control and automatic parking to platooning and completely autonomous drive. Some of the fuel saving benefits require full automation but others not.

A car that drives itself could be as good as the best drivers 

Some people get better fuel economy from their cars than others. Avoiding unnecessary braking and acceleration and good gear/clutch management are the main factors. Theoretically, a car that drives itself could be as good as the best drivers. The AA reckons that their drivers improved consumption by an average of 10% by adopting eco-driving tricks [2]. However, many of these were not to do with the actual driving – like combining trips and reducing weight by taking out heavy stuff you don’t need.

Cruise control is already available on many cars and there are claims it saves fuel but some people think it increases consumption. I expect it depends on your normal driving style. I can think of a few who would definitely benefit.

A convoy of four trucks 4m apart gives fuel savings up to 15%

Aerodynamic drag is by far the biggest energy loss at high speeds on the flat. When vehicles drive very close together, the lead vehicle suffers more or less the usual drag but the vehicles behind benefit from being in its wind shadow. However, when I say close I do mean close – at most one vehicle length. This is called platooning and there are a number of research teams working on it in Europe and elsewhere. The New Energy and Industrial Technology Development Organisation in Japan has tested a convoy of four trucks programmed to drive just four metres apart. The fuel savings were about 15% [3].

Platooning can be achieved with an active human driver at the front and the autonomous vehicles following it using wireless communication so they brake and accelerate together. Ideally the distance between the vehicles is constant, as if they were mechanically linked. If trucks could safely do this follow my leader trick in towns as well as on the motorway then truck companies could reduce the number of drivers.

For the average driver, platooning would have to be a more dynamic affair.  If you are on a motorway and you happen to meet a car that can platoon with you then you could request to join up. You would have to take turns being in front to share the fuel savings.

Theoretically platooning could cut down on congestion because you can get better throughput of vehicles at high speed. This would only work if a significant number of vehicles were platooning - and if the reduced congestion did not encourage more cars onto the road.

Driving closer together would increase road throughput and reduce congestion

Theoretically if all cars were autonomous they could drive closer together everywhere and still be safe because the car’s reaction time would be quicker than the average driver. Driving closer together would increase road throughput and reduce congestion. Since traffic jams are pretty bad for fuel consumption, savings would be made.

However, this only works as long as reduced congestion does not encourage more traffic. Historically, new road capacity gets clogged very quickly [4]. Also, autonomous vehicles will enable longer journeys and more users. Consider your daily commute. If you didn’t have to drive you could do something useful while travelling so you would not mind a longer journey. Also if cars were completely autonomous then they could be used by more people: the elderly with failing eyesight, the very young, people who can’t afford the expense of learning to drive and getting insurance... Making car travel more accessible would be a social benefit but it will not help with fuel savings.

When you want a car it could come to you and offer you a door to door service 

If cars were completely autonomous it would make car sharing more convenient because when you wanted a car it could come to you and offer you a door to door service. More car sharing reduces fuel consumption because car sharers tend to use cars less often. Part of the reason for this is that if you own your own car the extra cost of each trip is fairly low but if you rent a car by the hour then each use has a significant cost.

There are advantages to car sharing. In particular you can request a different sort of car depending on what you intend to do with it – some small and easy to park for the weekly shop, an estate to fetch a wardrobe from Ikea or a comfy cruiser to take the family on a camping holiday.

However, there are still lots of reasons for having your own car. For example your car has your mess in it and no-one else’s: your hairy dog blanket, raincoats, wellies, umbrellas, sweets, emergency nappies for the baby, books and toys to amuse the kids... You probably use your car as a giant handbag and you certainly would not want to share that with anyone else.

There are other reasons why autonomous vehicles would be a good thing

There are other reasons why autonomous vehicles are potentially a good thing. They ought to reduce accidents. Full automation would increase mobility for people who can't drive themselves, plus it could reduce stress and increase productivity for people who need to drive a lot because they can do something else while traveling. But fuel savings are pretty unlikely because any gains from more efficient driving are likely to be lost through more people using them and traveling longer distances.

[1] Google: driverless cars now have better understanding of city driving (Guardian) 28 April 2014
[2] Eco-driving advice (AA) March 2013
[3] Truck platoon demo reveals 15% bump in fuel economy (SAE International) May 2013
[4] New roads create new traffic (Campaign for better transport)


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