Transport has a major effect on health and the environment because it uses most of the world's petroleum. This leads to health problems and deaths caused by air pollution, such as harmful gases and tiny particles in the air. It also contributes greatly to climate change by releasing carbon dioxide. In the transport sector, road vehicles are the biggest source of climate change effects.

In developed countries, rules have helped reduce pollution from individual cars. However, more cars on the road and more use of each car have canceled out these improvements, a situation known as the Jevons paradox. Scientists have studied many ways to lower emissions from road vehicles. Different types of transport use energy and produce emissions at different levels, which has led experts to suggest shifting from air and road transport to rail and human-powered options. They also support using more electricity and improving energy efficiency in transport.

Other environmental effects of transport include traffic jams and the growth of cities designed around cars, which can harm natural areas and farmland. Reducing transport emissions worldwide is expected to improve air quality, reduce acid rain and smog, and slow climate change. Health issues linked to transport include noise from vehicles and carbon monoxide in the air.

Although electric cars help reduce carbon dioxide emissions when they are used, many cities are now focusing on improving public transport, biking, and walking. Creating neighborhoods where people can reach daily needs within 20 minutes encourages physical activity and reduces car use and pollution. Some policies involve charging drivers extra fees for driving in busy areas during peak hours.

Types of effects

Transportation has major effects on the environment in several areas: climate change, air quality, noise, water quality, soil quality, biodiversity, and land use.

Transportation is a major source of greenhouse gas emissions (GHGs) worldwide. About 30 percent of these emissions come from transportation in many countries. In some regions, the percentage is even higher. According to the International Energy Agency (IEA), transportation causes more than one-third of all carbon dioxide (CO₂) emissions globally.

In the United States, transportation methods are the biggest source of GHGs, contributing to 47 percent of the increase in total emissions since 1990.

Other environmental effects of transportation include traffic congestion and the spread of cities that focus on cars. These can harm natural habitats and farmland. Reducing transportation emissions worldwide may improve air quality, reduce acid rain, smog, and help fight climate change.

Transportation emissions also affect human health. Studies have found that exposure to traffic emissions can affect how long a pregnancy lasts and may impact the baby's growth inside the mother.

Noise pollution and carbon monoxide emissions harm the environment directly and indirectly. Indirect effects can be more serious than immediate ones, even though people often think the direct effects cause the most harm. For example, tiny particles from incomplete burning in car engines are not directly linked to respiratory or heart problems but contribute to other issues. While environmental effects are often listed separately, they can also combine to create larger problems.

Mode

The table below compares the emissions from different transportation methods used for passenger travel in Europe:

Air travel emissions depend on the length of the flight. Longer flights use more energy because of the high energy needed for takeoff and landing, but they are more efficient per mile traveled compared to very short flights. CO2 emissions from air travel range from 0.24 kg CO2 per passenger mile (0.15 kg/km per passenger) for short flights to 0.18 kg CO2 per passenger mile (0.11 kg/km per passenger) for long flights. Scientists are concerned about the growing trend of frequent, long-distance air travel, which increases environmental and climate impacts. This trend could undo progress made in making aircraft more efficient. In 2008, climate scientist Kevin Anderson warned that even with slower growth in UK air travel and reduced emissions in other sectors, aviation could account for 70% of the UK's allowed CO2 emissions by 2030.

Aircraft emissions at high altitudes have a greater effect on the Earth's climate than emissions at sea level. This is because emissions from aircraft include other greenhouse gases, such as methane (CH4), nitrogen oxides (NOx), which contribute to ozone (O3), and water vapor. In 2005, aviation was responsible for 4.9% of all human-caused radiative forcing, which affects the Earth's heat balance.

Cycling produces very low carbon emissions and has a small environmental impact. A European study of thousands of city residents found that daily travel-related CO2 emissions averaged 3.2 kg (7.1 lb) per person, with cars contributing 70% and cycling 1% (including the entire lifecycle of vehicles and fuels). People who cycled regularly had 84% lower lifetime CO2 emissions from all travel compared to those who did not cycle. Those who switched from driving to cycling as their main travel method reduced their daily CO2 emissions by 7.1 kg (16 lb). Regular cycling was most effective in reducing emissions for commuting and social trips.

Switching from motorized to non-motorized travel can significantly lower emissions. A European study of nearly 2000 participants found that cycling one additional trip per day and driving one fewer trip per day for 200 days a year would reduce lifetime CO2 emissions by about 0.5 tonnes annually. This is a large portion of the average yearly CO2 emissions from transport, which range from 1.5 to 2.5 tonnes per person, depending on location.

When burned, unleaded gasoline produces 8.91 kg (19.6 lb) of CO2 per gallon, while diesel produces 10.15 kg (22.4 lb). International agreements do not count CO2 emissions from ethanol, so gasoline with 10% ethanol is considered to produce 8.02 kg (17.7 lb) of CO2 per gallon. New light-duty vehicles sold in the US in 2017 had an average fuel economy of 24.9 miles per gallon (MPG), resulting in about 0.36 kg (0.79 lb) of CO2 per mile. The Department of Transportation's MOBILE 6.2 model, used to predict air quality, assumes an average of 20.3 mpg for all cars, resulting in about 0.44 kg (0.97 lb) of CO2 per mile.

In Europe, the European Commission required that all new cars registered after 2015 must not emit more than an average of 0.13 kg (0.29 lb) of CO2 per kilometer. By 2021, the target was reduced to 0.095 kg (0.21 lb) of CO2 per kilometer.

On average, inner city buses emit 0.3 kg (0.66 lb) of CO2 per passenger mile (0.18 kg/km per passenger), while long-distance bus trips emit 0.08 kg (0.05 kg/km per passenger). Some calculations add 10% to the total trip distance to account for traffic jams, detours, or stops.

On average, commuter rail and subway trains emit 0.17 kg (0.37 lb) of CO2 per passenger mile (0.11 kg/km per passenger), while long-distance trains emit 0.19 kg (0.42 lb) of CO2 per passenger mile (0.12 kg/km per passenger). Some calculations add 10% to the total trip distance to account for detours or delays.

Electric trains contribute less pollution because power plants, which are more efficient than diesel engines, generate the electricity used. Even with energy lost during transmission, electric motors are more efficient than internal combustion engines. Efficiency improves further with systems that save energy when trains slow down.

Trains have many parts that can create noise, such as wheels, engines, and non-aerodynamic cargo. This noise can reduce property values near railways. Since 1979, US diesel locomotives have been required to be quieter than 90 decibels at 25 meters away. This noise is harmless to most animals, except for horses, which may become startled.

Railway cargo can cause pollution. Materials like iron ore, coal, soil, or aggregates carried in boxcars can release nitrogen oxide, carbon monoxide, sulfur dioxide, or hydrocarbons into the air. Liquid pollution can occur from fuel spills into water sources or from human waste discharge.

Building railways in natural areas changes the landscape through cuttings, embankments, dikes, and stilts.

Shipping

Sewage can be released into water sources from several places, such as wastewater treatment plants, runoff from farms with animals, and boats. These releases can harm water quality, which negatively affects water life and increases dangers to human health. Although sewage releases can harm all water environments, they may cause more problems in marinas, slow-moving rivers, lakes, and other waters where water moves slowly. This can lead to the growth of invasive species that push out other species, harming the environment and local businesses.

Ship emissions have major environmental effects. Many ships travel internationally between ports and are not seen for long periods, causing air and water pollution during their trips. Greenhouse gases from ships reduce the ozone layer's ability to block harmful UV rays. Sulfur and nitrogen from ships react in the air to form sulfate and nitrate. Emissions of nitrogen oxides, carbon monoxide, and volatile organic compounds (VOCs) increase surface ozone and methane oxidation, which harms the ozone layer. The impact of ship emissions on chemicals like NOx, CO, O3, OH, SO2, HNO3, and sulfate is studied using a model called Oslo CTM2. This model helps scientists understand how these chemicals spread globally and how their levels change throughout the day. Weather data, such as wind, temperature, rain, and clouds, used in the model, comes from a weather prediction system.

Shipping Emissions Factors:

The road haulage industry contributes about 20% of the UK's total carbon emissions each year, second only to the energy industry, which contributes about 39%. Road haulage uses a lot of fossil fuels and produces significant carbon emissions. Heavy goods vehicles (HGVs) are responsible for nearly 20% of all emissions from this industry.

Influence of e-commerce

In recent years, large retail companies have focused on online shopping, called e-commerce. Many now offer fast shipping, such as 2-day delivery. These options help buyers receive products quickly, but they also create problems for public roads and the environment. While online shopping can reduce a person’s carbon footprint compared to driving to a store, this is only partly true. Online shopping uses less energy than driving to a physical store and returning home because shipping can use economies of scale, which means saving energy by moving more items at once. However, these benefits are reduced when items are packaged separately or when customers buy from multiple stores instead of shopping in one place. M. Sanjayan, CEO of Conservation International, explains that two-day delivery increases the number of polluting vehicles on the road.

In the 2010s, consumers wanted fast delivery. A 2016 survey by UPS found that 46% of online shoppers left items in their cart because shipping times were too long. The same survey showed that 1 in 3 shoppers considered delivery speed when choosing where to buy. However, a 2024 McKinsey survey found that US consumers now care more about cost, reliability, returns, and sustainability than speed. Despite this, average delivery times have decreased by about 40%, from 6.6 days in 2020 to 4.2 days in 2023.

To reduce waste, customers should avoid returning items because returns on standard shipping undo the environmental benefits. Since 2009, UPS deliveries have increased by 65%, leading to more trucks on the road and higher carbon emissions. A technology called truck platooning helps reduce these effects. This system allows trucks to communicate with each other about speed, reducing traffic congestion and saving fuel by 10% to 20%.

The growth of e-commerce has increased the use of inefficient packaging. Companies have tried to reduce environmental harm through changes to their business models, including:

• Lockers: Customers can pick up or return packages from lockers in cities, solving the "last mile" problem of delivering to homes.
• Omnichannel retail: Consumers can buy online and pick up items in stores, such as IKEA’s in-store pickup options.
• Electric vehicles: Companies are using electric trucks to reduce emissions.
• Returns channels: More options for returning items, such as using in-store returns with packaging.

Mitigation of environmental effects

Sustainable transport is a way to move people that causes less harm to the environment for each person or for each mile traveled, or that can carry more people. Common examples of sustainable transport include trains, bicycles, and walking.

A Road-Rail Parallel Layout is a design that helps reduce the harm to the environment when building new transportation routes. This design places railway tracks next to highways. For example, in 1984, the Paris–Lyon high-speed rail line in France used about 14% of this layout, and by 2002, the Cologne–Frankfurt high-speed rail line used 70% of this layout.

Humidity and ventilation can help reduce air pollution in enclosed spaces. Air pollution is often higher inside subway systems because of the friction and braking of trains. Pollution is usually lower inside buses compared to cars or subways.

Mitigation does not always require large changes, such as building new roads. People can help by choosing to walk, bike, or take short trips instead of driving. A trip that includes walking, taking a bus, and riding a bicycle may be counted as a single transit trip. Many studies about transportation investments often miss the real problems caused by more cars, such as more parking needs, more traffic accidents, and higher costs for drivers. These studies also often overlook the benefits of using other types of transportation. Most models used to plan transportation do not consider how adding more road space can worsen traffic and overestimate how helpful new highways are. Transportation planning tools, like average traffic speeds, delays from traffic, and how well roads work, focus on how easily people can move rather than how well people can reach places they need to go.

Climate change is an important factor for 67% of Europeans when choosing where to go on vacation. Younger people, especially those under 30, are more likely to think about how their travel choices affect the environment. In 2022, 52% of young Europeans, 37% of people aged 30–64, and 25% of people over 65 said they would travel by plane. About 27% of young people said they would visit a faraway destination.

Europeans expect major lifestyle changes in the next 20 years. According to a 2021 climate survey, 31% of people believe most people will no longer own cars, and 63% think working from home will become common to reduce emissions. About 48% of people predict that individuals will be given specific energy limits to follow.