Road Pollution: How Do Roads Cause Environmental Harm?

Roads are a significant source of pollution, impacting the surrounding water, air, and land. Vehicle emissions, including exhaust fumes, leakage, and tyre wear, release pollutants such as carbon monoxide, nitrogen oxides, and fine particles, which contribute to respiratory issues, cardiovascular diseases, and cancer risks. Additionally, road construction materials, de-icing salts, and pavement wear further add to pollution levels. The transportation sector is a major contributor to greenhouse gas emissions, and road congestion exacerbates pollution by increasing vehicle emissions. The impact of roads on the environment extends beyond tailpipe emissions, underscoring the need for rethinking travel methods and infrastructure design.

Vehicle emissions

Fine particles, including those from exhaust fumes and tyre and brake wear, are another concern. These particles are toxic due to their composition, shape, and size, penetrating deeper into the body the finer they are. They have been associated with oxidative stress, respiratory and cardiovascular disorders, cancers, and neurodegenerative diseases. While particle filters can mitigate these risks, fine friction particles will remain an issue even with the transition to electric vehicles.

Furthermore, vehicle emissions contribute to water pollution. Pavement pollution, such as oil from vehicles, can be washed into storm sewers during rain, eventually reaching nearby water bodies. Car tyres are also a significant source of microplastic pollution in oceans, and road salt used for safer winter conditions has led to elevated salt levels in freshwater ecosystems and drinking water.

While electric vehicles may reduce carbon emissions, the overall environmental impact of transportation extends beyond tailpipe emissions. The number of vehicles, their composition, and usage patterns significantly influence the environment. The transportation sector in the United States, for example, is responsible for a substantial proportion of greenhouse gas emissions.

Road congestion

Congestion on the roads increases travel time and exposure to vehicle emissions per traveller. The average annual travel delay for a commuter during rush hour trips in the US was 38 hours in 2005, across 437 urban areas. This extended time on the road means that drivers, commuters, and those living near roadways are at greater risk of exposure to harmful emissions.

Congestion also reduces the dispersion of vehicle-related pollutants. Vehicle-induced turbulence is dependent on speed, so lower speeds caused by congestion can increase pollutant concentrations from roadway sources. Congested roads can also lead to more frequent speed changes, including acceleration, deceleration, stopping, and starting, which further increase emissions.

The impact of road congestion on pollution is particularly notable in urban areas. In China, more than 80% of cities suffer from heavy traffic congestion, which has led to substantial changes in air pollutants, especially PM2.5 and O3. This has resulted in an increased air-pollution-related premature mortality rate. Similarly, a study of freeway and arterial road traffic in the US found that congestion on arterial roads led to a sharp increase in health risks for both on-road and near-road populations.

The environmental and health impacts of road congestion are extensive, and addressing this issue requires a rethinking of transportation systems and land use.

Road materials

Asphalt is a significant source of air pollution. When heated to 140°C, the temperature of road-paving, asphalt releases a large amount of semi-volatile organic compounds. Emissions from asphalt fall as the temperature decreases, but they remain constant and significant at 60°C, a typical summer temperature in Los Angeles. This suggests that asphalt could be a long-lasting source of pollution.

Freshly paved roads release a large number of molecules that contribute to particulate air pollution. In Southern California, it is estimated that asphalt could lead to between 1000 and 2500 tons of particulate air pollution, compared to 900 to 1400 tons from gasoline and diesel vehicles. While asphalt may not cause more total air pollution than cars, it is clear that it is a significant contributor to air pollution.

Asphalt can be substituted with other materials such as clay tiles for roofs and concrete slabs for roads. However, there are environmental trade-offs to consider. For example, producing concrete causes high emissions of greenhouse gases.

In addition to air pollution, roads also contribute to water pollution through runoff. Rain and snowmelt can transport oils and grease, heavy metals, and debris from roads into surface waters. High concentrations of sodium and chloride from snow runoff containing salt can kill fish and alter water chemistry. Fertilizers, pesticides, and herbicides can also be carried by rainwater, contributing to algal blooms, excessive plant growth, and eutrophication.

Tyre pollution

Tyres are made from a combination of natural and synthetic rubber, steel, fillers, and heavy metals such as copper, cadmium, lead, and zinc. The wear and tear of tyres during driving processes generate tyre and road wear particles (TRWP) that contribute to airborne non-exhaust emissions. These particles contain a range of toxic organic compounds, including known carcinogens, which can have detrimental effects on human health and the environment.

Research has revealed that tyre wear produces 36 milligrams of particles per kilometre, which is nearly 2,000 times higher than the average emissions from vehicle tailpipes. Moreover, tyre emissions from electric vehicles are 20% higher than those from fossil-fuel vehicles due to their increased weight and torque, which causes faster and more intense tyre wear. Tyre particles have been found to affect air quality and pollute water sources, with an estimated three-quarters of ocean microplastic pollution originating from car tyres.

The toxic chemicals released by tyre wear have been linked to various health risks, including heart, lung, developmental, reproductive, and cancer outcomes. Additionally, tyre particles have been detected in soil and water, impacting nearby aquatic ecosystems and contaminating groundwater. The environmental impact of tyre pollution is further exacerbated by the materials used in road repair, such as tar pitch in pavement sealers, which release pollutants into stormwater.

While the weight of vehicles and driving styles contribute to tyre wear, efforts are being made to address the problem of tyre emissions. Solutions such as rain gardens and electrostatic plates attached to tyres have been proposed to capture and reduce tyre particle pollution.

Water pollution

Roads are a significant contributor to water pollution. Water pollution from roads is primarily caused by stormwater runoff, which occurs when rainwater washes pollutants from road surfaces into nearby water bodies. This can include toxic metals, hydrocarbons, chemicals from vehicle fluids, tires, and paints, and other contaminants. The pollutants of most concern for aquatic environments and human health are metals, particularly copper and zinc, and polyaromatic hydrocarbons (PAHs) like benzo[a]pyrene.

During road construction, erosion gullies can form, leading to sediment runoff that smothers aquatic habitats and clogs waterways. Heavy metals, oils, and other toxic substances from construction traffic and spillage can be absorbed by the soil and carried with runoff water into lakes, rivers, and bays. Fertilizers, pesticides, and herbicides used during construction can also be carried by rainwater into water bodies, contributing to algal blooms, excessive plant growth, and eutrophication.

Road traffic is a major source of water pollution. Contaminants from vehicle fluids, tires, and paints are transported to watersheds and rivers, with higher levels found next to roads with heavier traffic. These contaminants include organophosphate esters, which are toxic to aquatic organisms, and triphenyl phosphate, which has been linked to neurotoxicity in fish. The increasing number of paved surfaces contributes to stormwater runoff, as rainwater is not absorbed into the ground but instead carries pollution into nearby streams and rivers.

To mitigate the impact of roads on water pollution, it is essential to implement runoff controls during road construction and maintenance. This can include installing runoff-control measures to reduce pollution during and after construction, such as using the Toggleblok valve to stop pollution runoff. Regular street sweeping and proper maintenance of vehicles can also help to reduce the amount of road dust and contaminants that end up in waterways.

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