A Massachusetts Institute of Technology (MIT) theoretical study suggests that small changes in roadway paving practices could reduce efficiency loss, potentially eliminating a half-percent of the total greenhouse gas emissions from the transportation sector, at little to no cost.  (One suspects that would also mean a half percent reduction in fuel use.)

Every time you hear a deep rumble and feel your house shake when a big truck roars by, that’s partly because the weight of heavy vehicles causes a slight deflection in the road surface under them. It’s enough of a dip to make a difference to the trucks’ overall fuel efficiency.

The findings are detailed in a paper published in the journal Transportation Research Record, by MIT postdoc Hessam Azarijafari, research scientist Jeremy Gregory, and principal research scientist in the Materials Research Laboratory Randolph Kirchain. The study examined state-by-state data on climate conditions, road lengths, materials properties, and road usage, and modeled different scenarios for pavement resurfacing practices.

Map shows the potential reductions in overall greenhouse gas emissions from the transportation sector, state by state, that could be achieved by policies emphasizing the use of stiffer road surfaces. The greatest potential gains are seen across the southern part of the country. Image Credit: courtesy of the researchers. Massachusetts Institute of Technology. Click image for the largest view.

They found that that one key to improving mileage efficiency is to make pavements that are stiffer, Kirchain explained. That reduces the amount of deflection, which reduces wear on the road but also reduces the slightly uphill motion the vehicle constantly has to make to rise out of its own depression in the road.

Looking to the future, Kirchain noted that while projections show a slight decline in passenger car travel over coming decades, they show an increase in truck travel for freight delivery – the kind where pavement deflection could be a factor in overall efficiency.

There are several ways to make roadways stiffer, the researchers said. One way is to add a very small amount of synthetic fibers or carbon nanotubes to the mix when laying asphalt. Just a tenth of a percent of the inexpensive material could dramatically improve its stiffness, they say. Another way of increasing rigidity is simply to adjust the grading of the different sizes of aggregate used in the mix, to allow for a denser overall mix with more rock and less binder.

“If there are high quality local materials available” to use in the asphalt or concrete mix, “we can use them to improve the stiffness, or we can just adjust the grading of the aggregates that we are using for these pavements,” said Azarijafari. And adding different fibers is “very inexpensive compared to the total cost of the mixture, but it can change the stiffness properties of the mixture significantly.”

Yet another way is to switch from asphalt pavement surfaces to concrete, which has a higher initial cost but is more durable, leading to equal or lower total lifecycle costs. Many road surfaces in northern U.S. states already use concrete, but asphalt is more prevalent in the south. There, it makes even more of a difference, because asphalt is especially subject to deflection in hot weather, whereas concrete surfaces are relatively unaffected by heat. Just upgrading the road surfaces in Texas alone, the study showed, could make a significant impact because of the state’s large network of asphalt roads and its high temperatures.

Kirchain, who is co-director of MIT’s Concrete Sustainability Hub, said that in carrying out this study, the team is “trying to understand what are some of the systemic environmental and economic impacts that are associated with a change to the use of concrete in particular in the pavement system.”

Even though the effects of pavement deflection may seem tiny, he said, “when you take into account the fact that the pavement is going to be there, with thousands of cars driving over it every day, for dozens of years, so a small effect on each one of those vehicles adds up to a significant amount of emissions over the years.” For purposes of this study, they looked at total emissions over the next 50 years and considered the reductions that would be achieved by improving anywhere from 2 percent of road surfaces to 10 percent each year.

With a 10 percent improvement rate, they calculated, a total of 440 megatons of carbon dioxide-equivalent emissions would be avoided over the 50 years, which is about 0.5 percent of total transportation-related emissions for this period.

The proposal may face some challenges, because changing the mix of materials in asphalt might affect its workability in the field, perhaps requiring adjustments to the equipment used. “That change in the field processing would have some cost to it as well,” Kirchain said.

But overall, implementing such changes could in many cases be as simple as changing the specifications required by state or local highway authorities. “These kinds of effects could be considered as part of the performance that’s trying to be managed,” Kirchain said. “It largely would be a choice from the state’s perspective, that either fuel use or climate impact would be something that would be included in the management, as opposed to just the surface performance of the system.”

This research could be a start of something significant. For now a 0.5% gain in efficiency isn’t going to light off a bunch of interest. But that notation of longer life for a roadway could have immense economic effect. Paved roads are expensive, not so noticed as much of the costs are in fuel taxes pre loaded into the pump as the “price.” At today’s gas and diesel prices its a major component of the “price.”

For now with so much focus on CO2 gas, often the real payoff is overlooked in the hope the global warming thing will grab attention. But really, cutting fuel taxes by some noteworthy amount and far longer lasting, less potholes roads is much more likely to seize the attention of road engineers and consumers. “Where might that data be?” we could ask.


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