Although the nearly 21 million miles of paved roads around the globe appear static, their environmental footprints are anything but set.
When studying all stages of a road’s life using a technique called pavement life-cycle assessment, it becomes clear that a pavement’s environmental impact doesn't end with construction. In fact, there are significant emissions associated with a pavement during its operational life, also known as its use phase.
Several factors, like the pavement quality’s impact on fuel efficiency, lighting, and its ability to absorb carbon dioxide through carbonation all contribute to this footprint. What’s more, these factors can vary depending on the pavement’s context, which includes the climate and the amount of traffic. This can make a pavement’s use phase impacts difficult to calculate.
In a paper published in the Journal of Cleaner Production, researchers at the MIT Concrete Sustainability Hub (CSHub) examine the use phase of pavements and calculate the influence of context on their environmental footprint. Their work finds that the use phase is highly context-dependent.
Where the rubber meets the road
Although the use phase can have a sizable environmental footprint, decisions made before a pavement is even constructed can influence the size of that footprint.
“It turns out that the design and maintenance of pavements indirectly impact the environment,” explains Jeremy Gregory, CSHub executive director and an author of the recent paper. “Some of these impacts include the way that pavements impact climate through their reflectivity, through the absorption of carbon dioxide over time through the paving materials, and by how they affect the fuel consumption of the vehicles that drive on them.”
This latter effect, called pavement-vehicle interaction (PVI), causes excess fuel consumption and is one of the greatest contributors to use-phase pavement emissions.
As its name suggests, PVI refers to the interaction between a vehicle’s tires and the road it drives upon. It is a multifaceted phenomenon.
The first, and most apparent, aspect of PVI is roughness, which refers to irregularities in the surface of the pavement. In addition to affecting ride comfort, roughness can have a significant effect on fuel consumption.
“The rougher a pavement is, the more energy dissipation there is in the shock absorber system of a vehicle,” explains Gregory. “A vehicle must then consume more fuel to overcome this additional energy dissipation. We refer to this as excess fuel consumption.”
Along with roughness, the second aspect of PVI is deflection. “Deflection has to do with very heavy vehicles, primarily trucks,” notes Gregory. “The weight of a truck makes a small indentation in the pavement so that the vehicle is always driving up a very shallow hill. Like roughness, deflection also causes excess fuel consumption.”
Since excess fuel consumption only decreases fuel economy by a few percentage points, it isn’t that noticeable to the average driver. But when factoring in the often thousands of vehicles that drive across a stretch of pavement every single day, these few percentage points add up. In the case of California, excess fuel consumption on highways totaled 1 billion gallons over five years.
While roughness and deflection contribute significantly to use-phase environmental impacts, another factor is also in play — a pavement’s context.
“When we look at the overall life cycle assessment of pavements, we find that the results are very context dependent,” says Gregory. “The context includes the climate the pavement exists in, the amount of traffic for that pavement, the type of pavement design, and also the maintenance and rehabilitation schedule that’s planned for that pavement in the future. All of those factors will combine to determine the environmental impact of a pavement.”
The authors of the paper selected nine different scenarios to study the impacts of these context-specific conditions. They analyzed pavements in four U.S. states with different climates — Missouri, Arizona, Colorado, and Florida. Within each climate zone, they then looked at roads with different traffic levels.
After studying the data, they found that traffic was the most significant factor affecting pavement environmental impacts.
“It turns out that for pavements with really high traffic loads, a much bigger fraction of their overall environmental impact is associated with the use phase and the excess fuel consumption of vehicles,” explains Gregory.
For example, interstates, which have the most traffic, also had the greatest use-phase impacts — as much as 78 percent of total life cycle impacts.
“On the other hand, for pavements that have much fewer vehicles that travel on them, most of the environmental impact is associated with the materials and construction,” reports Gregory. These kinds of less-trafficked roads, like state and rural highways, displayed lower use-phase impacts of 38 percent and 37 percent, respectively.
In addition to traffic, the design and maintenance of a pavement also influence its environmental footprint.
For example, since interstates see a lot of passenger vehicle traffic, the roughness of their pavements is their primary source of excess fuel consumption. If not regularly maintained, an interstate’s roughness might increase, leading to greater excess fuel consumption.
Since truck traffic is higher on rural and state highways than on interstates, the deflection of those pavements may have a greater impact on excess fuel consumption than roughness. To mitigate the effects of deflection, the pavement must be designed to be stiff enough to withstand a truck’s weight.
Climate also affects the environmental footprint of a pavement’s use phase. In colder climates, some pavements can deteriorate more quickly due to freeze-thaw damage, and therefore can have higher roughness. This increases the excess fuel consumption of vehicles on these pavements in cold climates.
In warmer climates, pavements made with petroleum-based materials deform more easily, which increases their susceptibility to deflection. In turn, trucks driving on these pavements in warm climates have greater excess fuel consumption.
Ultimately, this recent paper shows just how many contextual factors must be considered during a pavement’s use phase in order to make it as sustainable as possible. “It’s important to not assume any environmental impact for any given context,” explains Gregory. “You really have to run the numbers.”
The MIT Concrete Sustainability Hub is a team of researchers from several departments across MIT working on concrete and infrastructure science, engineering, and economics. Its research is supported by the Portland Cement Association and the Ready Mixed Concrete Research and Education Foundation.