The efficiency of the US transportation sector could be improved significantly by increasing the stiffness of road surfaces, US researchers have shown. Randolph Kirchain and colleagues at the Massachusetts Institute of Technology came to this conclusion through a detailed analysis of the road networks in each US state. Their findings could lead to a meaningful dent in the greenhouse gas emissions produced by US transportation, without the need for costly new technologies.
Although road surfaces may seem completely rigid when we walk on them, it is a different story for large vehicles. As they drive, the wheels of large vehicles compress and elastically deform road surfaces, creating temporary “valleys” from which they must continually escape. Even when driving on flat surfaces, these vehicles are always driving slightly uphill. Kirchain’s team calculates that this is results in an excess fuel consumption of over 2.5 billion tons across a 50-year period.
The researchers suggest that this problem could be alleviated by simply making roads more rigid – reducing the deformation produced by heavy vehicles. They examined several methods to achieving this goal: in one approach, they found that road stiffness could be increased by up to 93% by incorporating carbon nanotubes into various construction materials, at a proportion of just 0.1% by weight. Improvements could also be made by adjusting the sizes of the grains used in concrete mixtures, which would increase their densities. Yet perhaps their simplest method was to replace existing asphalt roads with more expensive, yet stiffer and more durable concrete.
Southern opportunities
Kirchain’s team then performed a state-by-state analysis of the current rigidity of US roads, accounting for factors including climate, road length and usage, and the properties of construction materials. They found that the potential to offset excess fuel consumption was highest in the southern states – whose roads are primarily made from asphalt, which is deformed particularly easily in their warmer climates.
Road freight: cutting the emissions load
The researchers calculated that by resurfacing 10% of the road network each year with the stiffer materials, the US could eliminate 18% of the greenhouse gas emissions associated with road deformation across a 50-year period. This corresponds to a 0.5% reduction in emissions across the entire US transportation sector.
The results present a robust basis for improving the efficiency of the US road network, with no need for technological innovations, novel construction materials, or unfamiliar manufacturing processes. In future work, Kirchain and colleagues will explore how further aspects could affect vehicle efficiency; including road roughness and reflectiveness, and the emissions associated with material production and demolition.
The work is described in Transportation Research Record.
