We’re overhauling our cars in the name of energy efficiency — why not our roads? 

This past November, the Federal Highway Administration (FHWA) launched a performance measure giving state Departments of Transportation (DOTs) and Metropolitan Planning Organizations (MPOs) a way to track greenhouse gas emissions associated with transportation. These moves follow the recent Bipartisan Infrastructure Law, which, among other investments, created a $6.4 billion program to fund carbon reduction strategies and support a range of projects to reduce emissions from on-road highway sources. 

The measure estimates highway tailpipe carbon emissions and compares them against a 2022 benchmark. This traces the effectiveness of state initiatives to encourage greater fuel efficiency and electrification — both of which reduce the impact of a mile driven — and to shift to transportation modes like public transit and biking, which lower the number of miles driven.  

However, this overlooks an important lever impacting transportation emissions: The performance of our road system. 

When thinking about tailpipe emissions, it is natural to focus on changing the vehicles themselves. In virtually every driving scenario, moving to a hybrid or full electric vehicle will reduce carbon emissions. However, the long-lived nature of modern vehicles means that full fleet transitions are measured in decades, not years. Most vehicles on the road today have internal combustion engines (ICEs). Based on current trends, the U.S. Energy Information Agency projects that in 2050, when 60 percent of the new vehicles sold are projected to be electric vehicles, most vehicles on the road will still likely be powered by ICEs. 

Achieving climate goals will require that we accelerate the transition to a fully electric fleet. It is important to note that while states can influence vehicle choice, ultimately the fleet transition depends on the decisions of individual consumers. However, investing in a higher-performance road system is a lever within state control that will improve the efficiency and carbon emissions of all vehicles, regardless of how each is powered. 

Smoother, stiffer roads allow cars to travel along it more efficiently. Every time a car tire traverses a bump, crack, or pothole, energy is wasted. These imperfections in the road cause energy to be dissipated through the vehicle’s suspension. As a result, the car now consumes more energy than it would have if driven the same distance on a smooth road. Road roughness wastes energy and increases emissions. 

The same is true of other road imperfections. For instance, roads bend under the weight of vehicles — especially trucks. Roads that bend more require more energy to drive along. We can relate to this phenomenon, called deflection, by noting that it takes noticeably more energy to walk along a sandy beach than a paved sidewalk; we sink into sand but rebound against pavement.  

Roughness and deflection are two components of a road’s use-phase emissions, which is one of several components of the life cycle of a road. This cycle begins when the materials used to make the road are created and continues when the road is constructed. As cars begin to use the road, we arrive in the use phase. The life cycle also includes road maintenance and rehabilitation, and ultimately concludes when the road is decommissioned, and its materials are recycled or disposed of.  

Achieving smoother, stiffer roads will require either more material in the form of thicker asphalt pavements or switching to other materials like concrete that provide more stiffness and maintain smoothness longer. In either case, producing those stiffer and longer-lasting roadway sections will generate carbon emissions. However, for highways, the emissions savings from better roads far outweigh the additional production emissions.  

Our research group, the MIT Concrete Sustainability Hub (CSHub), found that up to 78 percent of pavement life cycle impact comes from the use phase, and that the most important part of it is pavement-vehicle interaction, or PVI. PVI, which includes roughness and deflection, happens every time any vehicle drives on the road. With 286 million vehicles operating across the U.S. in the first quarter of 2023, it is evident that PVI is happening now and has an enormous impact. However, it is something on which we can act now to improve.  

We can make demonstrable improvements to road ride quality and environmental footprint with smart investments and long-term thinking. For instance, by prioritizing concrete paving for high-volume traffic roads, we can keep important thoroughfares smoother and stiffer for longer. In our case study of Missouri, we could reduce GHG emissions by 8 percent by adopting this strategy. 

In addition, when roads need maintenance, we should invest in treatment actions that will improve ride quality over the long term. More frequent road resurfacing and rehabilitation measures like diamond grinding are needed. Of similar importance, less frequent, long-lasting treatment like concrete overlays can provide long-term smoothness and dampen the negative impacts of deflection. Two examples created in a collaboration between the CSHub and the International Grooving and Grinding Association are the Fuel/Carbon Savings Calculator and the Rigid Pavement Savings Calculator. 

The new FHWA metric and Bipartisan Infrastructure Law are making great strides to reduce the greenhouse gas emissions of American road networks, and much credit is due to the lawmakers responsible.  

We should ensure that these programs help to build more sustainable physical assets — smoother and stiffer roads — to amplify the benefits of these investments. DOTs and MPOs should seek to not only lower tailpipe emissions by encouraging further adoption of electric vehicles and hybrids, but also invest in roads so that all vehicles, regardless of how they are powered, can travel along them more efficiently. As a start, we would challenge the FHWA to expand the current performance measure to include the carbon emissions from road performance. FHWA already collects all the information needed to generate that metric.  

Randolph Kirchain is director of the MIT Concrete Sustainability Hub, where Hessam AzariJafari is deputy director.