Image courtesy of Climate Visuals

TL;DR — Vehicle emissions comprise a significant portion of global air pollution, with vehicles responsible for a wide range of pollutants known to be harmful to human and environmental health — including particulate matter, nitrogen dioxide, volatile organic compounds, and ground-level ozone. Traffic-related pollution is made worse by idling and traffic congestion, which causes a build-up of pollutants in specific areas. While vehicle emission standards have made significant progress in reducing pollution levels over the last fifty years, there is still progress to be made. Policies such as the tightening emission standards and the implementation of the Ultra-Low Emission Zone (ULEZ) such as the one in London make a serious dent in air pollution —  and air quality monitoring helps us measure the benefits of these policies.

What is traffic-related air pollution?

Vehicle traffic is responsible for a significant amount of air pollution and greenhouse gas emissions. In fact, transport is estimated to contribute almost one-quarter of global energy-related CO2 emissions.

Research such as that from the Health Effects Institute (HEI) report links vehicle emissions to detrimental impacts on both air pollution and climate change. To learn more about the relationship between air quality and climate change as well as how air quality co-benefits can help drive climate action, read our blog here.

Data from the National Library of Medicine suggests that vehicle emissions have become the dominant source of air pollutants in many areas as traffic has increased and some other sources of emissions have decreased.

Traffic-related emissions are also responsible for some air quality standard exceedances.

From 2014 to 2020, just under two-thirds of all reported exceedances of air quality standards [in EU Member States’ air quality plans] were linked to dense traffic in urban centers and proximity to major roads, mainly due to emissions of nitrogen oxides (NOx)”

— European Environment Agency

The sources and composition of traffic-related air pollution

Vehicle emissions contain a wide variety of harmful air pollutants — including carbon monoxide, carbon dioxide, volatile organic compounds (VOCs), nitrogen oxides, and particulate matter, all of which have demonstrated negative impacts on human and environmental health.

Secondary pollutants, such as ozone and secondary aerosols, also occur as by-products of these pollutants and cause harm.

The exact pollutant composition from vehicle emissions depends on a variety of factors, such as vehicle type, age, maintenance conditions, type and quality of fuel, and the wear of parts like tires and brakes.

Particulate matter (PM) can come from both combustion sources — like vehicle combustion — as well as non-combustion sources such as tire and brake wear that stirs up these particles. Diesel exhaust is also a significant source of PM pollution.

To learn more about the harms of particulate matter air pollution, read our air quality measurements series blog here.

Nitrogen oxides are another major pollutant of concern.

Among the air pollutants gasoline and diesel engines emit are oxides of nitrogen—NO and NO2, generically abbreviated as NOx. Nitrogen oxides have harmful direct effects on human health, and indirect effects through the damage they do to agricultural crops and ecosystems. Vehicle NOx emissions have been regulated since the 1960s”

— The International Council on Clean Transportation

Read our blog here to learn more about nitrogen dioxide air pollution.

Volatile organic compounds (VOCs) are emitted from vehicle tailpipes and through fuel evaporation from cars, trucks, and buses. These emissions can also include certain toxic air pollutants such as benzene that have been linked to different forms of cancer.

Ground-level ozone, a secondary pollutant formed by the reaction of VOCs with nitrogen oxides in the presence of sunlight, is the main component of smog. In addition to decreased visibility, the pollutant has also been linked to respiratory irritation and reduced lung capacity.

Read our air quality measurements series blog on ground-level ozone pollution here for more.

Vehicle emissions also include greenhouse gases such as carbon dioxide that significantly contribute to climate change and increasing temperatures.

Tailpipe emissions from cars, trucks and buses account for over one-fifth of the United States' total global warming pollution; transportation, which includes airplanes, trains and ships accounts for around thirty percent of all heat-trapping gas emissions.”

— Union of Concerned Scientists

Learn more about how different air pollutants contribute to climate change here.

The impacts on air quality and public health

Vehicle emissions have serious consequences for air quality and resulting public health outcomes.

Vehicle emissions are regarded as a primary contributor to air pollution and related adverse health impacts. Heavy traffic congestion increases traffic flow and thus produces more O3 precursor emissions, leading to more adverse air quality issues.”

— Wang et al., 2023

One study focusing on specific regions of China found that the excessive amounts of PM2.5 and ozone pollution resulting from traffic congestion induce an additional 20,000 and 5,000 premature deaths in China, respectively. Those living in major cities had up to a 17.5% increased rate of premature mortality due to traffic congestion.

Studies have shown that these negative impacts especially affect drivers, commuters, and those living near major roads.

The causes of traffic air pollution

Major contributing factors to traffic-related air pollution are traffic congestion and vehicle idling. 

Congestion has significant impacts on air pollution concentrations. Stops at red lights and intersections with traffic signals tend to have significantly greater air pollution levels — up to 29 times higher PM concentrations as compared to open roads.

According to research from Atmospheric Environment, though drivers spent only 2% of their time passing through traffic intersections, these periods accounted for 25% of their air pollution exposure.

Traffic congestion also produces a buildup of pollutants that act as precursors to ground-level ozone.

Idling vehicles is also a significant portion of the problem.

Researchers estimate that idling from heavy-duty and light duty vehicles combined wastes about 6 billion gallons of fuel annually. About half of that is attributable to personal vehicles, which generate around 30 million tons of CO2 every year just by idling” 

— U.S. Department of Energy

Research also finds that idling is no longer necessary nor the best way to warm up cars and therefore should not be practiced.

One major concern when it comes to idling is school pick-up lines for children. Many groups campaign for caregivers to turn off their cars rather than idle because children can be exposed to high levels of air pollution — as vehicle emissions are more concentrated closer to the ground.

Find out more about the harms of idling here.

Urban design is also an important factor when it comes to reducing traffic-related air pollution. Because congestion and traffic intersections increase the build-up of air pollution, urban design may prioritize roads with fewer stops and where the speed limit is in a more optimal fuel consumption zone — which most research suggests is between 45 and 65 miles per hour.

Approaches to reducing air pollution from traffic

One primary way that traffic-related air pollution can be minimized is through the promotion of alternative transportation options, such as bus, metro, and rail.

Research from the UNEP notes that this is not a simple change to make.

This requires a fundamental shift in investment patterns, based on the principles of avoiding or reducing trips through integrated land-use and transport planning. Moreover it is necessary to shift to more environmentally friendly modes of transport and improving vehicles and fuels, which is seen as a priority to reduce urban air pollution and greenhouse gas emissions. In order to reduce volumes of traffic and emissions, regulations and standards, environmental friendly technologies and concepts for public transport and 'green cities' have to be implemented”

— UN Environment Programme

By improving existing public transportation systems through increasing investment and designing systems that best serve residents’ needs, cities and municipalities can encourage increases in ridership.

Past scenarios show us that favoring public transit over private vehicle use leads to significant gains in air quality. 

During the 2008 Beijing Olympic Games, for instance, most Beijing residents chose public transit or cycling as their dominant mode of transport because a large portion of private and business cars were restricted in use according to Olympic traffic management. Consequently, a noteworthy reduction of traffic flow was noticed during the Olympic traffic control days and on-road air quality improved significantly: the average reduction rates of PM10, CO, NO2, and O3 reached 28%, 19.3%, 12.3%, and 25.2%, respectively”

— Xia et al., 2013

A similar situation was observed during the 1996 Atlanta Olympic Games when the city saw 30% drops in peak daily ambient ozone levels and a resulting drop in asthma cases.

Encouraging sustainable mobility practices

One major practice that has been successfully put into practice in recent years is the Ultra Low Emission Zone (ULEZ) in the city of London, which restricts vehicles that do not meet certain emission standards from driving into zones of the city, or else they must pay a daily charge.

Research finds that the ULEZ has significantly decreased the number of older, more polluting vehicles that enter the city and reduced air pollution exposure levels. In fact, between 2016 and 2020, central London saw an almost 50% decrease in nitrogen dioxide air pollution — which was a five times faster reduction compared to the rest of the United Kingdom.

The graphic above from Streetlight Data highlights the impact that vehicle use has on climate change, particularly through the lens of carbon dioxide emissions. By working to make alternative forms of transportation, such as biking and light rail use, more feasible for residents, cities can get more cars off the roads. (Image source: Streetlight Data)

Alternative forms of transportation like biking and light rail have a significantly lighter carbon dioxide footprint, making them a much more sustainable choice for transportation as compared to car use.

Making cities more bike-friendly with more bike lanes that lead to central, highly visited parts of the city is a key part of increasing bike ridership as an alternative form of transport.

Monitoring strategies for traffic-related air pollution

Establishing air quality monitoring networks around traffic is essential to understanding pollution exposure.

Traffic-related emissions contribute to primary and secondary local, urban, and regional (background) pollutant concentrations against a background of similar contaminants emitted from other sources. Traffic emissions are the principal source of intra-urban variation in the concentrations of air pollutants in many cities; thus, population-oriented central monitors cannot by themselves capture this spatial variability” 

— Health Effects Institute Special Report 17

Hyperlocal, high-resolution networks in particular are crucial because they allow officials to understand differences in air pollution concentrations even over a small geographical area, as traffic-related air pollution can vary at such a small scale — such as between an intersection and a connecting part of an open road.

To learn more about a high-resolution, low-cost air quality monitoring network in action, check out the work of our partners at Breathe London here.

Intelligent transportation systems (ITS) are one strategy that has been gaining traction. ITS collect, analyze, and communicate data related to transportation in order to improve its efficiency, mobility, and safety and can also work to reduce impacts on the environment. ITS also equips transportation users with more information that helps them to reduce their travel time, travel more safely and comfortably, and minimize traffic problems they may encounter.

One such example is in the city of Glasgow, whose ITS equips commuters with a plethora of information about city buses, including their timing, seat availability, current location, the density of passengers in the bus, and more. ITS also functions to improve the attractiveness of public transport to residents.

Intelligent Transport Systems (ITS) are vital to increase safety and tackle Europe's growing emission and congestion problems. They can make transport safer, more efficient and more sustainable by applying various information and communication technologies to all modes of passenger and freight transport”

— European Commission Mobility and Transport

Environmental regulations and traffic policies

When it comes to government initiatives and legislation related to traffic air pollution, many governments have emissions inventories in place. These are based on complex emission models — such as the US EPA’s MOBILE6 model — that provide exhaust and evaporative emissions for a wide variety of air pollutants and greenhouse gases.

Widespread regulations are also in place to restrict the air pollution allowed from vehicles.

In the United States, the Clean Air Act of 1970 — and its later amendment in 1990 — originally gave the US EPA the ability to regulate air pollution emissions from transportation.

Since its advent, the Clean Air Act has significantly reduced traffic-related air pollution levels. Research finds that new passenger vehicles’ tailpipe emissions today are 98-99% cleaner than in the 1960s. Fuels are also cleaner — with sulfur levels being 90% lower today than before the legislation — and U.S. cities have cleaner air today despite an increased population that travels more miles.

Regulations in place have largely been effective at reducing vehicle-related emissions. Research finds that between 1998 and 2014, vehicle emission standards have cut emissions from cars and trucks by 90%.

VOC emissions in particular have dropped by over 99% in the last 50 years.

In general, tightening vehicle exhaust standards has decreased national emission levels each year. Research finds that for every dollar of compliance costs, over ten dollars are seen in environmental benefit.

Another potential strategy to reduce vehicle-related emissions is to get older, more polluting vehicles off the road. Some proposed ways to do so is to increase older vehicles’ registration fees to reflect their level of environmental damage. Research from Kleinman Center for Energy Policy shows that if this were put into policy, vehicle emissions would fall by one-third and many people would get rid of vehicles over twenty years old.

However, there are also issues when it comes to this strategy, so it is not such a clear-cut  solution.

Lower income households drive older and more polluting vehicles. Hence, increasing registration fees for older vehicles would burden these households. Tightening exhaust standards disproportionately increases prices of newer vehicles, which initially affects higher-income households more, and higher-income households also own more vehicles per person. Revenues from higher registration fees could be used for rebates or tax cuts to low income households to make the changes less regressive”

— Kleinman Center for Energy Policy 

Integration of environmental considerations in traffic planning

By acknowledging the gravity of the impacts that traffic-related air pollution has on human and environmental health, officials can more extensively integrate environmental considerations into traffic planning.

Environmental considerations can be made in many different aspects of transport plans. The plans should consider the impacts on existing residents and ecosystems, existing institutions such as schools and hospitals, agricultural lands, groundwater, and soil erosion, in addition to air quality. (Image source: NBM&CW)

Interested in measuring air quality for cleaner air, improved physical and mental health, and a healthier climate? Get in touch with our team to learn more about our Sensing-as-a-Service solution for governments, businesses, and community organizations, using our Clarity Node-S monitors and modules that do not depend on infrastructure like WiFi or power — making them especially resilient during environmental disasters.