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Would capping the number of years a car can be on the road reduce emissions? Not so much.


Would capping the number of years a car can be on the road reduce emissions? Not so much.

It’s an appealing idea, but a comprehensive new modeling study shows that the strategy only delivers for the climate in limited circumstances.
May 21, 2024

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Policies to encourage people to trade in their old cars for newer, more climate-friendly models only make a meaningful dent in carbon emissions when combined with other technological or policy changes, according to a new study.

Passenger cars and trucks are responsible for 17% of annual greenhouse gas emissions in the U.S. New technologies such as more fuel efficient, lightweight, and electric vehicles have the potential to reduce these emissions, but people don’t buy new cars very often—which makes getting lower-emissions technologies on the road a painfully slow journey.

This dilemma has led some to advocate setting a maximum number of years a car could be on the road or coming up with other incentives to get people to retire their vehicles earlier than they otherwise would.

But this strategy won’t get us very far, the new work shows. Without other changes to the transportation and power systems, simply capping vehicle lifespans will barely reduce overall carbon emissions at all.

The researchers used a computer model to assess the climate impact of cars and trucks across their whole lifespan, and estimate the effectiveness and cost-effectiveness of vehicle lifespan caps on reducing emissions. They analyzed nine different scenarios for how the U.S. passenger car and truck fleet and the technology that drives it could evolve between 2020 and 2050—for example, modeling the effects of smaller, lighter, or more fuel-efficient vehicles, and wide-scale adoption of hybrid, plug-in hybrid, or electric vehicles, as well as combinations of these changes.


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With no other technology changes, a cap on vehicle lifespans could reduce carbon emissions by at most 0.1%, the researchers report in the journal Environmental Research: Infrastructure & Sustainability. “Although vehicle lifespan caps can reduce [greenhouse gas] emissions from fuel use, these are offset by emissions from the associated increase in vehicle production,” they write.

What’s more, the scant emissions savings would come at the price of other environmental stressors, such as a 2-7% increase in demand for metals such as aluminum and steel.

But lifespan caps could contribute to reduced carbon emissions if they’re combined with other strategies, such as decarbonizing the electric grid, making vehicles more fuel efficient, reducing carbon emissions involved in vehicle production, and especially, wide rollout of battery electric vehicles.

For example, a 15-year lifespan cap with wide rollout of electric vehicles yields a 4% reduction in the vehicle fleet’s greenhouse emissions through 2050 compared to a business-as-usual scenario without a lifespan cap. This is about the same scale of climate benefit that could be achieved by making cars as lightweight as possible.

The biggest cuts in carbon emissions come when the lifespan cap applies only to conventional gasoline vehicles. “Under a technology specific lifespan cap the cap accelerates alternative vehicle adoption but keeps these alternative vehicles on the road for their full lifetimes,” the researchers write.

However, vehicle lifespan caps could be a costly strategy. The researchers determined that the lifespan cap scenario with widespread electric vehicle deployment would only be cost-effective under an optimistic set of assumptions, including the combination of lower cost vehicles, low production emissions, high material recycling, better fuel efficiency, and low-carbon electricity. Under these conditions the cost of averting a ton of carbon emissions via lifespan caps and EV deployment is roughly equal to high estimates of the cost of a ton of carbon emissions to society.

In addition, it would be very difficult to implement a hard limit on vehicle lifespans, except in authoritarian countries, the researchers say. “By contrast, a form of vehicle lifespan cap that relied on subsidies to encourage a desired rate of vehicle replacement may deliver lower cumulative [greenhouse gas] emissions but be more feasible to implement,” they write.

Source: Striepe M.C. et al.  “Are vehicle lifespan caps an effective and efficient method for reducing US light-duty vehicle fleet GHG emissions?Environmental Research: Infrastructure and Sustainability 2024.

Image: Based on a photo by Scott Umstattd on Unsplash.

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