While your car's tires may have intimate familiarity with the roads, most likely you ride atop them without much thought beyond how they'll get you from point A to point B. Besides providing you with a path around town, roadways enable efficient commerce. In fact, the United States so loves roads that it's built almost 4 million miles (6.4 million kilometers) of them, according to the U.S. Geological Survey.
There is, however, increasing concern about roads' impact on the environment. How harsh are roads on their ecological surroundings?
Mother Nature Knows Best
Nature provides the perfect means of processing storm water. Storm water seeps downward, traveling through purifying soil, rocks and other natural barriers before entering ponds, lakes and streams. Roads disrupt this natural cleansing by creating runoff, water laced with road-top residue (oil, rust, rubber, metals, gases and more) that's forced to take unnatural routes. The traveling liquid picks up toxins along its way, such as fertilizer and motor oil, and in its wake leaves high concentrations of toxins on vegetation, causes soil erosion, and too quickly enters natural waters without benefiting from nature's slow purification process.
Are some roads worse offenders than others? Let's look at the types of roads and their compositions.
The Dirt on Dirt
You might think the old-fashioned dirt road is harmless. But that's not so. Airborne dust from road creation and from wind and traffic compromises vegetation, erodes soil (increasing runoff), increases water sediment loads (degrading aquatic vegetation) and pollutes the air.
Controlling dust creates new problems. For years, petroleum-based coatings, such as emulsified asphalts and motor oil, were used to stop the dust from flying off the roads. More recently, the slightly less damaging calcium chloride has been used to grab moisture out of the air and weigh down the materials on the surface of the road, but its runoff is so environmentally unfriendly that its use may be illegal in some states, according to Grit.com.
Gravel: The Rocky Road
Gravel, composed of sand, silt or clay and varying sizes of broken limestone, quartzite and granite, is dumped then flattened onto a pathway to create a road. The good news is gravel is permeable, so water and pollutants drain downward. The bad news is that the bottom layers of the gravel wind up packed so tightly water can't flow through them. Polluted waters pool atop those impermeable spots way beneath the gravel's surface, making it difficult to identify those pools and dashing hopes of routing the pooled water through purifying pathways.
Gravel is dusty, too, but gravel roads are less traveled than paved roads and therefore accumulate and relay fewer pollutants.
Ninety-six percent of America's paved roadways are asphalt, a substance composed of molecules, mostly polynuclear aromatic hydrocarbons, extracted from crude oil at the refinery.
To build an asphalt road, the material (5 percent asphalt and 95 percent sand and gravel) is heated to 300 to 400 degrees Fahrenheit (148 to 204 degrees Celsius) in a truck's rotating hopper, poured over gravel and smoothed using a spreading machine. Asphalt hardens as it cools.
Asphalt is a surface water can't seep through, and runoff of chemicals such as zinc, copper, rust and cadmium, enters water tables, streams and rivers where that unfiltered runoff can poison aquatic life, taint drinking water and introduce E-Coli bacteria, too, making waters unsafe for recreation.
Are porous pavements a pollution solution?
Three types of permeable pavement systems hold promise for combating run-off: interlocking block, pervious concrete paving, and porous asphalt. Interlocking block roads use interlocking concrete blocks with small openings between permeable joints. These roads can be laid in interesting, stylish patterns in urban areas. Pervious concrete is a special concrete mix that uses fewer fine materials, such as sand, resulting in stable air pockets introduced into the mix, the same process as used when creating porous asphalt, according to BMPClean.org.
Asphalt is the most popular substance for U.S. roads, so the question is: Can porous asphalt prevent toxic run-off? The Georgia Asphalt Pavement Association (GAPA) says "yes." Porous asphalt roads also conserve water, facilitate natural cleansing, are economical and last for decades, it adds. And, with proper instruction, existing manufacturing plants can easily incorporate air pockets to mix porous asphalt. Construction methods vary; GAPA describes a stone bed of 18 to 36 inches (45 to 91 centimeters) at the bottom and porous asphalt on top. When it rains, porous asphalt roads more closely emulate nature; water drains downward into the stone bed, then slowly makes its way into the soil, where nature's processes take over.
The University of New Hampshire had great success with a porous asphalt parking lot built in 2004. In three years of measuring, it found no surface runoff. What's especially interesting is that runoff from adjacent structures can be directed into the porous asphalt stone bed, cleansing those waters as well.