Big, intractable problems like climate change don't have a single easy solution—if they did they wouldn't be intractable, after all. Rather, any successful strategy to reduce climate emissions will involve the additive effects of lots of smaller fixes. Not everyone is going to switch their car to a battery electric vehicle (EV) or even a plug-in hybrid EV, barring a government mandate. But there are applications where new technology can be both cost effective and good for the environment, like Wrightspeed's plug-in EV drivetrains for delivery trucks.

A paper published today in Nature Climate Change examines the effect to greenhouse gas emissions if another automotive niche converted to a more climate-friendly fleet: what might happen if, in 2030, US taxis were all self-driving, battery-powered EVs?

The authors, Jeffery Greenblatt and Samveg Saxena at Lawrence Berkeley National Laboratory, focus on autonomous taxis as having a viable business model. Many users and frequent trips make up for higher purchasing costs; indeed they point to Uber and Lyft as an analogous car-on-demand approach. While their paper looks at autonomous taxis, the calculations are mostly comparing current vehicle efficiencies with predictions for 2030. The benefits of being autonomous (as opposed to human-driven) are assumed to come through more efficient allocation of vehicles—sending the right-sized vehicle to the right customer, balancing use across the fleet optimally, traveling in packs, and so on.

They began by using US Department of Energy (DoE) statistics for different light duty vehicle efficiencies in 2014 and projected efficiencies for 2030, looking at both mile per gallon and carbon emissions per mile. They consider conventional internal combustion vehicles, hybrid EVs, battery EVs, and hydrogen fuel cell vehicles (for the final two, they also took into account 2030 projections from EPA and also California's various policies on reducing emissions, both of which are more optimistic than those of the DoE).

The Department of Energy estimates that even internal combustion vehicles should more than double in efficiency (and halve in CO 2 emissions) over the next 16 years. But battery EVs have the most promise, getting to 160mpge (miles per gallon equivalent) and emitting between 11 and 23 percent as much carbon as the 2014 conventionally powered fleet. While battery EVs offer the best improvements over conventional vehicles, they also cost more, which is why we don't expect the entire passenger fleet to switch en masse.

Not all journeys are equal; their efficiency can depend upon how many people are in the car. To factor this in, Greenblatt and Saxena used 2009 data that breaks down vehicle occupancy per mile traveled. These stats were used to calculate the actual energy savings possible for autonomous battery EV taxis, assuming an autonomous taxi travels 40,000 miles per year compared to a fleet average of 12,000 miles per year. Their weighted average (1.6 passengers per mile) suggests that autonomous taxis would use 45 percent less energy than the average light vehicle in 2030.

Total costs per year would also be lowest for battery EVs driven 40,000 miles a year. By contrast, in 2014 you'd need to drive 70,000 miles a year for a battery EV to work out cheaper than the alternatives.

Using current financing models and interest rates and data from New York City's taxi economy from 2005, they calculate that battery EVs are financially viable despite the higher up-front purchasing price. (Although we do have to wonder whether the salaries quoted for taxi drivers would still be relevant in a future where there were no more taxi drivers.) As with the promise of autonomous heavy goods vehicles, the societal benefit from fewer accidents and better efficiency may well do away with some types of employment in the same way that the automobile did for the buggy whip industry.

The study also gives us pointers to a larger transformation that may take place regarding how we use transportation in the future. If, as predicted, ride-sharing and autonomous vehicles do work out much cheaper and more efficient per mile traveled, how long before traditional models of vehicle ownership and utilization become anachronisms? Will we still need to own our own cars when we can summon Johnny Cab at the push of a button?

Nature Climate Change, 2015. DOI: 10.1038/nclimate2685 (About DOIs).