So policymakers may need to consider additional strategies to clean up transportation, experts said. That could include policies to buy back and scrap older, less efficient cars already in use. It could also include strategies to reduce Americans’ dependence on car travel, such as expanding public transit or encouraging biking and walking, so that existing vehicles are driven less often.

“There’s an enormous amount of inertia in the system to overcome,” said Abdullah Alarfaj, a graduate student at Carnegie Mellon University who led a recent study that examined how slow vehicle turnover could be a barrier to quickly cutting emissions from passenger vehicles.

That study suggested several options for speeding up the rate of turnover. For instance, policymakers could focus on electrifying ride-sharing programs like Uber and Lyft first, since those vehicles tend to drive more miles on average and get retired sooner.
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