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Smoothing Roadways and Cutting GHG Emissions

When is it time to re-surface a road or freeway? Ask drivers, and it’s when the road is uncomfortably bumpy and a full tank of gas doesn’t seem to go as far. Ask a cash-strapped state department of transportation and it often depends on available funding.

FALL 2012 — In recent decades, pavement management systems have been developed to minimize costs to drivers as well as to transportation agencies, like Caltrans, charged with keeping roads in good repair. That optimal frequency might be anything from five to 20 years, depending on a number of factors, including the amount of traffic, the rate of deterioration of the roadway, and the costs to users in terms of vehicle wear and tear and fuel consumption.

But with state laws mandating sharp reductions in greenhouse gas (GHG) emissions, University of California pavement experts realized a new type of pavement management system was needed, one that could help decision-makers lower tailpipe emissions as well as the environmental impacts of resurfacing roadways if emissions  were considered along with costs in the equation. 

About 94 percent of the two million miles of paved roads and highways in the U.S. are surfaced with asphalt, the rest with concrete. On the state highway network, approximately 75 percent have asphalt surfaces, and the rest are surfaced in concrete. Resurfacing worn roadways with asphalt produces large amounts of GHG emissions, especially when taking into account the entire supply chain—such as the fuel used to extract stone, sand or gravel, to the production of bitumen, a petroleum product, to the energy used to transport all these materials to the site and operate construction equipment.
 
Replacement of broken concrete slabs also produces GHG emissions due to the same mining of the aggregate and construction and to the decomposition of calcium carbonate (limestone) to calcium oxide (lime) and carbon dioxide to produce cement.
 
So delaying these overlays and slab repairs for as long as possible not only saves agencies money but would seem to curtail the amount of harmful emissions released into the atmosphere.
 
On the other hand, cars, trucks, and buses use more fuel when they travel on rougher roads, which also result in greater amounts of tailpipe emissions.
 
So given these complex effects and trade-offs, is it possible to devise an optimal schedule for repaving California’s network of roads and freeways that takes these often conflicting costs and requirements into account?
 
Ask a team of engineers from UC Berkeley and UC Davis and they’ll tell you they’re on track to find the sweet spot—the optimal resurfacing frequency—that best balances user and agency costs and reduces GHG emissions for a variety of roads and freeways throughout the state.
 

Rolling resistance 

 
In 2009, after a competitive process, faculty at the Institutes of Transportation Studies were awarded a major five-year grant from the UC Office of the President to find solutions to California’s most vexing transportation-related problems:  increasing congestion, oil use, air pollution and greenhouse gas emissions.  The ITS Multicampus Research Project proposal included a number of collaborative research topics on technology, system management, and land use planning solutions to improve transportation sustainability.
 
One of those research topics involves a team of researchers at ITS Berkeley and ITS Davis: Samer Madanat, chair of UC Berkeley’s Department of Civil and Environmental Engineering (CEE) and an expert in pavement management; Berkeley CEE Professor Arpad Horvath, an expert in life-cycle analysis; UC Davis CEE Assistant Professor Alissa Kendall, who specializes in life-cycle modeling applied to transportation and energy systems as well as construction materials; and UC Davis CEE Professor John Harvey, a pavement expert and co-director of the UC Pavement Center, which is equipped with a variety of sophisticated pavement testing and analysis equipment.
 
These faculty members have guided graduate students at each campus who have worked on the project.
 
The UC Davis team has concentrated on developing life-cycle analysis models to evaluate the effects of rolling resistance and varying levels of traffic on greenhouse gas emissions. In a paper recently published in the Journal of Cleaner Production, the Davis team reveals early findings in case studies.
 
Different types of roadways deteriorate at different rates and in different ways, which affect the tailpipe emissions that emanate from them and the environmental costs of resurfacing them.
While rolling resistance—the effects of a road on vehicle mileage—may have little impact on the additional fuel needs of an individual car or truck, it becomes far more significant when tens of thousands of vehicles on a heavily-used freeway must use more fuel due to a rough road.
 
Different types of roadways—congested urban streets, winding rural roads, steep mountain roads, little-used state highways, and congested freeways—deteriorate at different rates and in different ways, which affect the tailpipe emissions that emanate from them and the environmental costs of resurfacing them.
 
The UC Davis team is trying to group them by certain characteristics and apply the life-cycle models they’ve developed for them. Each case study will take into consideration traffic conditions, climate, and truck loads, explained Harvey.
 
“Generally speaking, on high volume roads resurfacing pavement to make it smoother and reducing the negative effects of rolling resistance will result in fewer emissions,” added Kendall. “But what if it’s a highly congested freeway where cars are already crawling? In that case, will it make any difference in terms of emissions if the road is smoother?”
 
Harvey, Kendall and their student researchers will conduct dozens more case studies in the coming months to characterize and group the various types of roads throughout the state.
 

Finding the Pareto frontier

 
The Berkeley team, led by Madanat and Horvath, is concentrating on building a decision framework for local and state governments that reflect the life-cycle costs of resurfacing and life-cycle environmental emissions.
 
The difficulty is that one is measured in dollars and the other in CO2 emissions.
 
“Unless you could put a price on a ton of CO2 you cannot really combine the two into some giant minimization,” explained Madanat.
 
To solve this oranges-and-apples conundrum, the researchers turned to a method named for an early 20th century Italian economist, Vilfredo Pareto.
 
“The Pareto curve, or frontier, gives us a way to represent these kinds of trade-offs where you have two objectives that are conflicting and that are measured in different units,” explained Madanat.
 
“If the x axis is in metric tons of CO2 and the y axis is in dollars, we can find a set of points, where a point represents an interval in years between two successive overlays, which we join by a curve,” Madanat explains.
 
In a paper  to be published by the Journal of Infrastructure Systems, former ITS Ph.D. students Jeffrey Lidicker and Nakul Sathaye, along with Madanat and Horvath, describe the process of finding the optimal Pareto frontier for two different types of roadways: a ten-lane section of Interstate 80 near the Bay Bridge and State Route 13 (Ashby Avenue), a mostly two-lane arterial.
 
Factoring road roughness and its effects on the fuel consumption of vehicles weighing different amounts, as well as the supply chain emissions that result from resurfacing, the researchers determined that the interval between overlays for I-80 should fall between 15 and 22 years. Repaving at, say, 14 years produces a large amount of emissions as well as high costs. Increasing the interval to 15 years improves both objectives; fewer emissions and lower costs. But between 15 and 22 years, any improvement in one objective—emissions, for example, comes at the expense of cost. Similarly, an improvement in cost will come at the expense of greater emissions.
 
When Lidicker and his co-authors looked at Ashby Avenue they found the curve looks similar, but the magnitudes of costs and emissions are almost four times smaller.
 
By plotting these points along the curve, the researchers can show decision-makers in city halls, county seats, and Sacramento basically what their dollars will buy.
 
“If they are willing to spend very little money on pavement resurfacing, they can end up with much higher emissions,” said Madanat. “If they spend more, they can bring those emissions down—if they operate along the Pareto frontier.”
 
“What is interesting about this is that by making that choice, they are saying in effect, we in California are willing to spend this many dollars to reduce greenhouse gas emissions by one ton.”
 
The goal of the researchers, who are halfway through the five-year period of the grant, is to determine a Pareto curve for every type of highway in California based on the data the UC Davis researchers are gathering.
 
But there is one more confounding element: state budget constraints, and often even tighter constraints on local government road funding.
 
If Caltrans had unlimited funding, the agency could simply do what is optimal for every road and the entire network of roadways would be operating at optimal efficiency. But given the reality of available funding, that is impossible.
 
So as the final step in their research efforts, the UC Berkeley team will use the Pareto frontier approach to determine what is optimal for the entire network of California roads.
 
“When we are done, we will be able to present a guide for allocating limited funding each year in a way that is optimal for the entire network—even though it may not be optimal for every segment of roadway every year,” said Madanat.
 
And that will give the state the biggest bang for every buck spent on resurfacing the state's roads.
 
--Christine Cosgrove
--Pareto frontier graphic/Darren Reger