California is in a fight over fuel economy with Washington. But what California state officials and its drivers don’t realize is the quality of the state’s roads has an important impact the amount of fuel we use. It’s not just potholes either – the smoothness and stiffness of the roads we drive on determines how hard our vehicles have to work to reach their destination.
We can see how traffic slows us down and increases emissions, but a dynamic called pavement vehicle interaction can have almost as big an effect because it impacts everything from greenhouse gas emissions to our annual budget for gasoline. All vehicles, whether gasoline, diesel or electric, use energy to move — but some of that energy is being wasted. On roads with poor surface conditions or subpar structural properties, vehicles consume more fuel than what’s needed to move. Researchers are now focusing in on that excess fuel consumption by taking a hard look at PVI.
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PVI boils down to three factors — roughness, texture and deflection — all of which combine to determine how much bang you get from the bucks you spend at the gas station. The first component, roughness, relates to how bumpy or smooth the road is. Commonly seen and felt via cracks and potholes, roughness is a big determinant of the performance of passenger vehicles.
The next factor is texture — the abrasiveness of the road surface — which relates to vehicle traction when surface conditions are wet.
The final component is deflection, or the bending of pavement under the weight of a vehicle. This is a big factor for trucks and other heavy vehicles. Deflection is a condition determined by the initial construction of a road, and ultimately depends on pavement design. Think of the difference between walking on sand versus a sidewalk — vehicles work harder when the deflection of a pavement is greater.
Excess fuel consumption and air emissions can be significantly reduced by building stiffer roads and maintaining smoother pavement. Road stiffness is particularly relevant for 40-ton trucks, where research shows that lessening the impacts of deflection could generate up to 4 percent in fuel savings. When you consider the amount of fuel consumed by trucks each year, that 4 percent in savings could quickly add up.
Altogether, 2 million tons of savings in carbon dioxide emissions each year are achievable by addressing road stiffness nationwide. As more efficient hybrid and electric power-trains become increasingly more common —helping to eliminate vehicle engine losses in and of themselves — the role of PVI in the excess energy consumption of trucks could increase to as much as 8 percent.
As with many other issues, once again, California is the laboratory for the nation that is helping us illustrate the real-world impact of PVI. Using data collected by the California Department of Transportation through the use of GPS and ground-penetrating radar, researchers conducted an analysis of the state’s entire 50,000 lane-mile system, and found that PVI accounted for 1 percent of overall fuel consumption on California highways. At a scale as vast as the California highway system, that one percent amounted to 1 billion gallons of excess fuel consumption over a five-year period due to PVI factors. Researchers identified 1 million tons of CO2 emissions associated with excess fuel consumption, and the analysis reveals that only small portions of the entire interstate network is often responsible for a large part of its total greenhouse gas emissions, meaning rehabilitation of those few lane miles would bring significant environmental improvements.
As we continue to make strides with vehicle fuel efficiency, it’s worth keeping an eye on PVI. Actions that improve road design and conditions can reduce vehicle fuel consumption and emissions. As well, it becomes an important factor for state officials who should want to improve transportation maintenance, planning and budgeting so taxpayers can get the most effective bang for their transportation buck. California is a leader in this space and continues to be aggressive searching for ways to maximize infrastructure spending with greenhouse gas reductions. The evidence is in and environmental and economic paybacks are clear.
Dr. Jeremy Gregory is the executive director of the Concrete Sustainability Hub at the Massachusetts Institute of Technology.