Image provided by Matthew Henry via Unsplash.
TL;DR: Transportation is a major source of nitrogen oxides, particulate matter, volatile organic compounds, and carbon dioxide. Public transportation reduces these emissions by moving more people with fewer vehicles and, in some cities, by running on cleaner fuels. In San Francisco, New York, and London, public transit systems lower greenhouse gases and other pollutants, preventing premature deaths and improving air quality.
The transportation sector is responsible for approximately 45% of total nitrogen oxides (NOx) emissions in the United States. This sector emits particulate matter (PM), volatile organic compounds (VOCs), and air toxics like benzene and formaldehyde. The burning of fossil fuels not only emits harmful air pollutants and carbon dioxide (CO2) but also indirectly leads to the formation of ground-level ozone (O3) through atmospheric chemical reactions. Even the wear and tear of tires generates fine particulate matter, which can bypass the body's natural defenses.
These air pollutants harm human health, with transportation emissions resulting in hundreds of thousands of deaths worldwide every year. Also, carbon dioxide and ground-level ozone both trap heat, contributing to global warming.
With these negative impacts in mind, cities turn to public transportation as a way to mitigate these air pollutants. To what extent is this solution effective, and what are some useful things to keep in mind when implementing public transportation in urban areas?
Public transportation’s role in reducing air pollution
Public transportation reduces air pollution because it is a more efficient way of transporting people than individual vehicles. While individual cars can only carry up to a handful of people, buses, trains, and other forms of public transportation can carry dozens, hundreds, or even thousands of passengers. Opting for public transportation over driving reduces the number of individual cars on the road, improving air quality.
Compared to driving alone, taking public transportation reduces carbon dioxide emissions by 45%. In the United States, public transportation is estimated to save 37 million metric tons of carbon dioxide from being released into the atmosphere annually.
Public transportation also reduces other air pollutants, including nitrogen oxides, particulate matter, and volatile organic compounds. These reductions can prevent many premature deaths from lung and cardiovascular diseases.
San Francisco, New York, and London all have robust public transportation. It is worth taking a closer look at how these cities’ public transport and other initiatives work to improve air quality.
San Francisco’s public transportation
San Francisco’s public transportation consists of the San Francisco Municipal Railway (Muni) and the larger Bay Area Rapid Transit (BART). The Muni consists of streetcars, cable cars, light rail, electric trolley coaches, and hybrid buses. San Francisco’s first cable cars were in use as early as 1873. The BART, a heavy-rail public transit system that connects San Francisco with the rest of the Bay Area, was formed in the mid-1900s.
By 2017, more than 52% of all trips to, from, and within the city used transit, bicycling, or walking rather than car travel. Consistent with this progress, by the same year, annual greenhouse gas emissions had decreased by 28% below 1990 levels despite a 19.5% population increase within that time frame.
San Francisco’s Muni uses renewable fuels with a low carbon intensity. So, not only is this public transport system more efficient by taking cars off the road, but it also largely runs on clean energy. Because of this, the Muni accounts for only 2% of the city’s transportation-related emissions. San Francisco’s public transit system has had incredibly positive results for both public health and the environment.
New York’s public transportation
New York City uses the Metropolitan Transportation Authority (MTA). The MTA is the largest transportation network in North America, serving 15.3 million people across the state of New York and Connecticut. The MTA includes buses, railways, and subways.
New York City’s public transportation system began with horse-drawn omnibuses in 1827. Although there was briefly an air-powered subway in lower Manhattan from 1870-1873, the first real subway system in New York was built in the early 1900s. Today, New York City houses a transportation system that is as extensive and robust as it is iconic.
Only about one out of every four New Yorkers uses a car to commute around the city. Residents rely on public transportation more than on private vehicles. This is not surprising considering that traffic is highly congested in parts of New York. The city’s public transportation provides residents with a quicker way to move around and, by extension, more economic opportunities.
The MTA also reduces air pollution by reducing the number of vehicles on the road. It works alongside New York’s congestion pricing to reduce car emissions. Every year, MTA transit riders avoid at least 20 million metric tons of greenhouse gas emissions from being released into the atmosphere.
Even with all these benefits, there is still room for improvement. New York City’s subway system has been criticized for having highly polluted air due to poor ventilation and harmful particulate matter. However, some progress has been made above ground as MTA’s bus fleet transitions to cleaner fuels and engines, resulting in fewer nitrous oxide emissions.
London’s public transportation
Transport for London (TfL) operates London's public transportation system, which includes the Tube (London Underground), London Overground, buses, trams, riverboats, and main roads. This network links to the National Rail, providing train connections throughout the United Kingdom.
The world’s first underground railway opened in London in 1863. Since then, London’s underground network has expanded to include 272 stations. During the 2016/2017 period, 1.37 billion journeys were made on the Tube. London’s aboveground railway network predates the Tube, and the city’s riverboat transportation stretches as far back as the 1500s.
In 2015, on-road diesel vehicles contributed 46% of the transportation health burden in London. In that same year, approximately 1,500 premature deaths in the metropolitan London area were attributable to ambient fine particulate matter (PM2.5) and ozone from transportation tailpipe emissions. Public transportation helps mitigate this issue.
London’s extensive fleet of about 9000 buses has low or zero tailpipe emissions. The buses rely on clean energy, reducing nitrogen oxide emissions. Like New York, the air in London’s underground network contains particulate matter. London’s Tube frequently accumulates dust (PM10) containing metal particles, organic matter, and minerals. However, TfL has invested millions of pounds in initiatives to reduce the dust and monitor air quality throughout the underground network.
In addition to public transportation initiatives, London’s ultra-low emission zone (ULEZ) has significantly decreased nitrogen dioxide (NO2) and fine particulate matter (PM2.5) emissions across the city, safeguarding public health.
The role of air quality monitoring moving forward
Public transportation plays a key role in building safer, more sustainable cities. Air quality monitoring supports this effort by providing the data needed to assess how well transit systems reduce pollution. It helps pinpoint areas where public transport or other measures are most needed and tracks the results of policy decisions over time.
Partner with Clarity to build your own air quality network and find the right initiatives to reduce air pollution.