Poorly maintained roads are a primary source of vehicle wear-and-tear – and can lead to hefty repair bills for drivers. What may be less obvious, however, is that potholes and uneven tarmac raise fuel consumption, as engines struggle against greater resistance. The result is a marked increase in greenhouse gas emissions.

"The additional environmental impact for a single car or truck is small, but these highways see very large traffic volumes," Darren Reger of the University of California, Berkeley told environmentalresearchweb. "When aggregated over the total number of vehicles that travel over these roadways, roughness can become quite significant."

Last year, Samer Madanat, Arpad Horvath and colleagues at Berkeley performed preliminary research into the effect of road resistance on the environment; they found that there is always a difficult mixture of trade-offs. Resurfacing can be expensive for highway agencies while being cost-saving for vehicles, which experience less wear-and-tear. On the other hand, resurfacing requires the use of materials such as concrete whose production can be environmentally damaging, yet the resultant roads will be smoother, requiring less fuel consumption.

Currently, according to Reger, highway agencies take into account aspects such as cost and driver comfort but not the environment. "The current policies are set by determining allowable thresholds," he explained. "These thresholds can be based on things such as perceived user comfort. We have not seen pavement management systems that also include environmental considerations."

Now Reger, working with Madanat and Horvarth, has built on the preliminary study to show how a perfect balance between all the trade-offs – known as the Pareto frontier – can be reached. Named after the Italian economist Vilfredo Pareto, the frontier denotes a curve on a cost–benefit graph where it is impossible to, for example, reduce total emissions without raising total costs, and vice-versa.

The team began with a computer model that took into account pavement deterioration, the effect of roughness on fuel consumption and greenhouse gas emissions related to resurfacing. Into this model they inputted data such as traffic volumes, pavement condition and road geometry from California’s highways. The model allowed calculation of the best times to resurface under a certain emissions budget; by repeating the calculation with different emissions budgets, the researchers built up the Pareto curve one point at a time.

Californian transportation agency Caltrans has started to explore different ways of reducing emissions due to the state’s recent adoption of a bill that sets out new emission targets. The Berkeley group found that some road sections managed by Caltrans were on the Pareto curve, while others were far off.

"The results were not surprising, but we were pleased that they demonstrated both positive and negative aspects of current pavement practices," said Reger. "It was good to be able to put a visualization to the recent policy change to show that we’re heading in the right direction, but also that there is room for improvement."

Reger believes that the general framework he and his colleagues have developed to prioritize the Pareto frontier could easily be incorporated into current policy, whether the emissions aspect is included or not.

"This simply optimizes the current way in which agencies already operate," he said. "With respect to including emissions into pavement-management systems, it’s possible that agencies will be pressed to consider these factors with more detail in the near future. When looking at potential ways to save greenhouse-gas emissions, changing the resurfacing policy should be considered along with other options, and is often the more economical choice in terms of dollars spent per metric-ton of carbon-equivalents saved." The study is published in Environmental Research Letters (ERL)

Related links

• Economically and environmentally informed policy for road resurfacing: tradeoffs between costs and greenhouse gas emissions, Darren Reger et al 2014 Environ. Res. Lett. 9 104020
• Darren Reger

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