An automated Chrysler minivan designed by self-driving technology company Waymo traverses a residential street. Credit: Waymo

The transportation sector is the single largest contributor to greenhouse gas (GHG) emissions in the US. It makes up 28% of the total, according to the recent GHG inventory completed by the Environmental Protection Agency; the rest comes mostly from electricity production (27%), industrial processes (22%), commercial and residential activity (12%), and agriculture (10%).

Automated vehicles (AVs), if they’re electrically powered, could put a big dent in those transportation emissions. Proponents argue that driverless cars could also provide other benefits, such as fewer accidents and less traffic congestion. And if used similarly to ride-hailing services such as Uber and Lyft that offer discounted trips to riders who carpool, AVs could lower the number of vehicles on the road, reducing emissions further.

But like ride-hailing services, AVs also motivate people to travel more. If they become widespread before being powered by clean energy, AVs will drive emissions higher. To really address climate change, decision makers must implement policies for AVs that alleviate congestion and incentivize carpooling.

Policies can manipulate three variables—CO 2 per gallon, miles per gallon (MPG), and vehicle miles traveled (VMT)—to encourage reductions in GHG emissions from transportation. That can be modeled as

GHG emissions = CO 2 per gallon MPG × VMT .

Better fuel-efficiency standards increase the MPG that vehicles can achieve. But that approach is incremental at best and, unfortunately, is now being weakened in the US: Later this year, fuel efficiency standards will be updated, and the latest proposal from the National Highway Traffic Safety Administration would improve fuel economy by only 1.5% annually, compared with the 5% per year that has been mandated since 2011.

The VMT annually on US highways has increased by about 250% since 1960, a trend that the National Climate Assessment projects will continue for the foreseeable future. Study after study shows that people hold on to their ingrained behaviors, which makes it unlikely that VMT will be reduced simply by asking people to abandon their cars for mass transit, bicycling, or walking.

Electric AVs powered by alternative energy sources would decrease emissions from the transportation sector by lowering or even eliminating the CO 2 per unit of vehicle fuel. Such a transformation would require electricity production to quickly and concurrently switch to some combination of solar, wind, nuclear, and other fossil fuel–free sources and be scaled up to meet the new demand.

It’s an ambitious goal, though climate scientists, economists, engineers, and other experts have many ideas for how to reach it, as summarized in the United Nations’ Fifth Assessment Report on climate change mitigation. Some of those include carbon taxes or cap-and-trade markets (see Physics Today, December 2019, page 28), land-use changes, sustainable investment in developing countries, and negative emissions technologies (see the article by David Kramer, Physics Today, January 2020, page 44).

Storing electricity is another technological challenge (see Physics Today, June 2013, page 26). The current generation of lithium-ion batteries for vehicles relies on cobalt, and there’s not enough of it to replace all of the vehicles on the road with electric ones (see Physics Today, December 2019, page 20).

Besides clean energy, some ethical problems, such as the trolley problem, should be solved. And technical issues remain too. For example, city driving presents far more risks—bicyclists, pedestrians, and vehicles that make frequent stops—than driving on highways, so more real-world driving by AVs will be necessary to make them safe.

But even if AVs can be electrified with clean energy, there’s a behavior issue. Because they lower cost and other barriers to travel, AVs are likely to encourage more driving. Consider the 2018 study on the behavior of households that were offered a free chauffeur service for 60 hours over a seven-day period, akin to a ride-hailing service or a future driverless car. The study participants’ VMT rose by about 80%.

In the aggregate, ride-hailing services exacerbate traffic congestion, and AVs could do the same. A study by consultants Fehr & Peers that focused on six large metropolitan areas found that 2–13% of total VMT is attributable to Uber and Lyft. According to a 2017 report from the Institute of Transportation Studies at the University of California, Davis, the convenience of ride-hailing services leads people to choose that option instead of mass transit or zero-emissions methods such as bicycling or walking, even for short trips.

Without access to Uber or Lyft to complete a two-mile trip, most people in a recent survey said they would choose transportation options that use less fossil fuels (green). Credit: Adapted from R. R. Clewlow, G. S. Mishra, Disruptive Transportation: The Adoption, Utilization, and Impacts of Ride-Hailing in the United States, UC Davis (October 2017)

Policies that reduce emissions and alleviate congestion from ride-hailing services are already being experimented with and should be applied to AVs. For example, to mitigate gridlock, Chicago mayor Lori Lightfoot changed the city’s ride-hailing tax for 2020 from a flat rate of 72 cents per trip to a location- and passenger-based surcharge. For trips starting and ending in downtown, the fee is $3 for a vehicle carrying one passenger and $1.25 for shared trips. At least some of the $40 million in estimated annual revenue from the tax will go toward improving Chicago’s mass-transit infrastructure.

California’s Clean Miles Standard, a statute put into effect on 1 January, will establish annual GHG reduction targets for ride-hailing services that are based on the miles traveled by passengers. The California Air Resources Board says on its website that the regulation is designed to decrease GHG emissions, increase the use of zero-emissions transportation options, promote carpooling, and provide a low- or zero-emissions framework through which to introduce AVs to the roads.

Other places have incorporated high-occupancy or toll lanes on heavily trafficked interstate highways. A project in northern Virginia completed in 2012 allows vehicles with three or more people to travel at no charge in an express lane of I-495, whereas other vehicles pay a toll whose rate depends on traffic conditions. The approach saves commuters time, and four similar projects for other local stretches of interstate highway have been completed over the past few years.

Chicago has implemented a tax to encourage users of ride-hailing services to carpool for downtown trips. Credit: Dondon83/PD

Electric AVs could effectively reduce emissions if they’re guided by policies like the ones in Chicago, California, and Virginia. A 2019 Union of Concerned Scientists report on the effects of AV deployment in Washington, DC, laid out “the three strategies that we think are the difference between the good outcomes and the bad outcomes,” says study coauthor Jeremy Martin. It recommends policies that encourage AVs to service mass-transit centers, promote carpooling, and electrify AVs. Those changes would help counter the increase in VMT by AVs and would position them to decrease GHG emissions as the electricity industry transitions to alternative, sustainable sources of clean energy.

A strategy to decrease transportation emissions with AVs depends, of course, on when they become broadly available. If the research, transportation, and regulatory communities can strategically and closely collaborate, “we could easily be inside of five years” for widespread AV adoption, says Curtis Walker, a meteorologist and AV scientist at the National Center for Atmospheric Research in Boulder, Colorado. But he says that if societal buy-in and political and regulatory processes prove challenging, “we are a decade or more away.”