“Travel budget” Needed to Meet Climate Goals

By Rayla Bellis

The transportation sector currently accounts for the largest share of carbon emissions in the U.S., so averting climate change will require the U.S. to drastically reduce transportation emissions. A recent study looked at what it would take to meet the U.S.’s carbon reduction target of 80% or greater for passenger vehicle travel by midcentury. The study found that even with a high level of electric vehicle adoption, the U.S. will need to stay within a “travel budget,” or maximum total miles of vehicle travel, to meet existing climate targets.

As the researchers note, the transportation sector is in the early stages of several significant interconnected transitions, including eventual widespread adoption of electric vehicles (EVs), autonomous vehicles, and evolving shared mobility options. These transitions have the potential to impact travel demand, energy use, and emissions in uncertain ways. However, there are some constraints to that uncertainty—for example, EV adoption is limited by how quickly people purchase new cars, so even in an optimistic scenario it will take time for the fleet to fully turn over. EVs currently remain a small percentage of both new sales (2.2%) and the total fleet of vehicles (0.43%).

Using those constraints to limit possible scenarios, the authors examined the requirements for meeting carbon reduction targets by modeling the changes needed in vehicle electrification, the carbon intensity of electricity, and travel demand. They found that if U.S. VMT stays at current levels (about 3 trillion vehicle miles per year), reducing carbon emissions from passenger vehicles by 80 to 90% will require 1) reducing the carbon intensity of electricity generation to essentially zero so that EVs aren’t contributing to emissions, and 2) electrifying 67% to 84% of travel by midcentury.

They also found that we will need to keep total miles of travel within a maximum travel budget to meet our targets at any realistic scale of EV adoption in the U.S. The more we drive, the greater EV adoption will need to be. For example, if we can reduce VMT to 1991 levels (about 2 trillion miles), we will only need to electrify 45% to 48% of travel to meet an 80 percent emissions reduction target. If VMT increases to 4 trillion, meeting that 80 percent target will require electrifying 73% to 79% of travel. The authors suggest that meeting emissions goals will require greater emphasis on policies to reduce total travel demand, including local policies around land use and housing, walkability and community design, and parking.

Photo credit: ikkoskinen via Flickr, unmodified. License