There are four million miles of roads stretching across America, per the American Road and Transportation Builders Association (ARTBA). That means sustainable construction could make a big impact on the environment. But only a small fraction of our roads are handled by the federal government, leaving state and local governments responsible for managing roads with limited budgets. They need to choose a material that is inexpensive, simple to build with and easy to repair. This is why asphalt, the ubiquitous pavement made from a sticky petroleum-based material, is used so frequently. But how can we move toward a sustainable plan for the future of roads and the health of our planet?
Understanding How Roads are Constructed
Traditionally paved roads are made of layers of sandy material capped with a surface layer of pavement. Author Sid Perkins explains in the Proceedings of the National Academy of Sciences (PNAS) that the top layer, called asphalt pavement, is a mixture of gravel, sand and other fillers that are stuck together with a black sticky petroleum byproduct.
Asphalt is flexible, so the pressure of vehicles can create dips and subtle dimples in the road over time. Plus, the natural expansion and contraction of groundwater below the road often gives way to cracks and seasonal potholes. Transportation crews are constantly working in a cycle of repairing and resurfacing roads, with 19.4% of our highways in need of repairs, according to the ARTBA.
Other more durable materials, such as concrete pavement, are too expensive upfront. Perkins explains that departments of transportation make construction decisions based on short-term costs without factoring in the cost of maintaining roads made of flexible materials.
Environmental Impacts of Traditional Roads
There has been plenty of attention on vehicles and their impact on the environment. Stricter vehicle emission standards and an uptick in electric and hybrid vehicles have helped to reduce automotive pollution. Now that those numbers have improved, it’s easier to see that the roads themselves are part of the problem.
The process of creating asphalt involves heating bitumen, a form of petroleum, to make it soft and sticky. Asphalt production is one of the biggest energy-consuming industries, according to World Highways. Not only that, but the heat and carbon dioxide emitted during production can be harmful to the environment.
Softer roads are also bad for fuel efficiency, especially as they continue to sag over time. Although road dips and dimples may appear subtle, they require vehicles to expend more energy to climb out of them, decreasing fuel efficiency and increasing carbon emissions. Per Hedges & Company, with 289.5 million registered vehicles in the United States projected in 2021, small dips in fuel efficiency quickly add up. And for tractor-trailer trucks that transport heavy goods, the dips are even more severe.
The dark asphalt itself also absorbs heat from sunlight and then emits fumes that contribute to fog, especially when it heats up on sunny days. PNAS reports that road surfaces may release more organic compounds than vehicles.
Alternative Composition and Production
So, what happens when all that cracked, old pavement gets replaced? The good news is that 200 million tons of old pavement are recycled each year, with much of it being reused in the new roads, according to ARTBA. This is a decent start, but there are more improvements in the works. The switch to more sustainable construction doesn’t have to be an all-or-nothing approach. Instead, we can make incremental improvements.
An alternative method of production called warm-mix asphalt can be produced at a lower temperature than traditional pavement, which helps to minimize the energy and fumes. Plus, we can mix different aggregates and binders. PNAS explains that researchers are considering replacing some of the traditional asphalt ingredients with:
Gravel and sand from wastewater
Incorporating these materials into the mix could help to reduce the use of natural resources, such as limestone, and keep waste out of landfills.
Aside from recycled materials, researchers are also investigating the use of lighter-colored materials. This would help to minimize the absorption of heat from the sun — which could help to reduce urban “heat islands” caused by pavement — and reduce emissions from the road materials themselves.
Pavement doesn’t have to be so soft. PNAS reports that stiffer pavement could cut “dimple” related emissions by 18% over 50 years.
Some municipalities are already starting to incorporate porous (also called pervious) concrete, which allows stormwater to filter through the pavement instead of running off along the sides of the road. This could:
Increase road safety, because roads are not as wet and slippery
Reduce the need for road salt, which gets into groundwater
Filter out pollutants that contribute to pollution in water
Future of Roads
As we brainstorm better solutions to repair and improve our roads, we can also look beyond the pavement. Some researchers are working on ways to make roads do double duty by incorporating energy into the roads themselves. In the future, instead of stopping at a gas station to fuel up, drivers could charge their vehicles while they drive.
For example, World Highways describes roads in the Netherlands that generate solar power via glass-covered photovoltaic cells. Meanwhile, researchers in Israel are developing a piezoelectric energy road, which uses the vibrations of vehicles driving along the road to convert mechanical energy into electrical energy, according to Business Insider. With this technology, electric coils installed under roads can transmit energy to electric vehicles, so they can charge as they go without the need for electric vehicle charging stations.
Clearly, there are many ways to consider more sustainable construction for the future of roads. The immediate barrier is that transportation infrastructure is currently based on short-term costs, and regular asphalt is the widespread, cheap and easy solution.
But new methods could also lead to a whole new set of problems. Different aggregates or binders could create new aerosols that could pollute the air. Plus, as PNAS points out, reflective coatings could create a glare that could distract drivers.
More research is needed, but with so many roads connecting our widespread country, even small steps toward sustainable construction could help to protect the environment.