Waste heat from cars, together with sealed road and car park surfaces, contributes to the generation of urban heat. This affects the quality of life and poses a health risk. As temperatures rise, it is time to act.

VCÖ-Factsheet "How traffic increases urban heat stress" PDF Download

Between 1991 and 2020, the average number of days with temperatures above 30 degrees Celsius in Austrian cities more than doubled compared to the period from 1961 to 1990.¹ High temperatures pose a serious risk to health, especially for children, the elderly and people with health problems. A total of 511 deaths were associated with heat in the summers of 2021 to 2023.² Sealed traffic areas and the heat generated by cars exacerbate heat stress in cities on hot days. To counteract this, we need to unseal our cities and create more green spaces and water bodies to cool the environment.

Car traffic multiplies heat stress

Waste heat generated by traffic accounts for up to 30 per cent of anthropogenic heat emissions in cities, making it the second largest anthropogenic heat source after buildings.³ In Vienna, for example, car traffic alone generates up to three times as much waste heat every day as the body heat of the entire population.⁴ Roads store heat and release it into the atmosphere during the night. Heated cars parked on the street reduce night-time cooling. Pollutant emissions increase heat build-up and impede air circulation.^5,6

Heat poses many health risks

Heat stresses the human body and can lead to cardiovascular problems, heat exhaustion or heat stroke and may trigger heart attacks and strokes. The elderly, children and people with chronic underlying medical conditions are particularly vulnerable to the effects of heat.^7,8 Heat also contributes to the spread of air pollutants such as particulate matter and ozone.⁹

Prolonged heat waves are particularly harmful to health, as the body is not able to recover sufficiently due to the lack of cooling during the night.¹⁰ Studies predict that there will be between 1,000 and 3,000 heat-related deaths per year in Austria by the middle of the century.¹¹ Night-time cooling is particularly important, as for many people night-time ventilation is the only way to reduce indoor temperatures. Night-time temperatures that are too high may disrupt sleep.¹² Fatigue leads to poor concentration and an increased risk of accidents.¹³

Urban heat inequity

Lower-income individuals tend to live in more densely built neighbourhoods with less green space, which makes them more vulnerable to higher temperatures.¹⁴ Income, age and gender are key indicators of how much people are affected by heat.¹⁵ Low-income individuals are three times more likely to suffer heat-related sleep problems than higher-income individuals. People over 65 are almost twice as likely to be affected as those under 65. Older people on low incomes are about ten times as likely to be affected as the rest of the population.¹⁶

Urban heat island effect due to surface sealing

Transport is responsible for 43 per cent of soil sealing in Austria.¹⁷ Especially in urban areas, where cooling green spaces, water bodies and air circulation are typically lacking, the high proportion of sealed surfaces results in significant temperature differences compared to less sealed areas. The so-called heat island effect exacerbates urban heat by trapping solar radiation in sealed surfaces, along with heat emissions from car traffic and buildings.¹⁸ Temperatures in large cities can be up to ten degrees Celsius higher than in rural areas.¹⁹

Waste heat from cars makes cities even hotter

When fuel is burned or electricity is drawn from the battery, heat is released to the surroundings. Therefore, all factors that affect the efficiency of a vehicle, such as type of drive, vehicle weight, engine power, driving style and speed, have an impact on the heat released. In Vienna, car traffic generates between 13 and 20 gigawatt hours of waste heat every day. This is about three times the body heat emitted by the entire population of Vienna. As electrical drives are much more efficient than internal combustion engines, electrifying vehicle fleets will help reduce the waste heat generated by car traffic.²⁰ Ideally, this will also reduce the number of kilometres driven. In addition, a shift to even more energy-efficient modes of transport, including public transport and active mobility such as walking and cycling, will reduce waste heat.

Parked cars delay cooling

Even when parked, cars still release waste heat into the environment, raising the ambient temperature. The shade underneath parked cars is beneficial only if the vehicles are white or light-coloured, as those heat up less in the sun than the road surface. Cars painted in dark colours will heat up just as much as the pavement.²⁰ Dark asphalt absorbs and stores much more heat than gravel or green areas. Therefore, parked cars on sealed surfaces increase the heat island effect. Parked cars also prevent the road surface from cooling down at night.²¹ A study has shown that for every ten additional cars parked on narrow streets, the temperature rises between 0.5 and 1.6 degrees Celsius.²²

Heat changes travel behaviour

People experience high thermal discomfort at temperatures above 25 degrees Celsius.²³ They will often try to avoid travelling or, where possible, travel in air-conditioned means of transport. Leisure travel is more likely to be affected by weather conditions than business travel.²⁴ The most weather-sensitive mode of transport is cycling. Studies have shown that the proportion of cyclists increases at a temperature of up to 25 degrees Celsius and decreases above 25 degrees Celsius.²⁵

Rise in energy demand creates a vicious circle

While the proportion of active mobility decreases on hot days, the number of car trips often increases at the same time. As temperatures rise, the use of air conditioning in both buildings and vehicles leads to increased energy consumption. This adds to the heat, which in turn leads to travel patterns that further increase greenhouse gas emissions, creating a self-reinforcing vicious circle.²⁶

Mitigating urban heat through sustainable mobility

Promoting sustainable and energy-efficient mobility helps mitigate the heat island effect. Austria‘s 2024 Heat Protection Plan focuses on the communication and information about different heat levels and recommends the creation of local heat protection plans. This is where we need to step in and develop a strategic national heat protection plan that includes proactive and effective measures prioritised by long-term impact, together with monitoring mechanisms for their implementation. Traffic-related heat mitigation measures, such as traffic calming or unsealing parking spaces, should be more widely promoted.

Katharina Jaschinsky, VCÖ ‑ Mobility with a future

„In order to improve and maintain the quality of life in cities, action is needed to reduce traffic-related heat stress. This includes promoting unsealing and shading measures and making energy-efficient mobility more attractive.“

VCÖ-Factsheet "How traffic increases urban heat stress" PDF Download

Content research: Paul Pfaffenbichler, University of Natural Resources and Life Sciences (BOKU), Institute of Transport Studies
VCÖ is responsible for the content and editing of the VCÖ factsheet. The content does not necessarily reflect the views or opinions of the supporting institutions. This factsheet was financed by the Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and supported by the federal provinces of Upper Austria, Styria and Tyrol.

Klimaaktiv advises and supports relevant stakeholders, decision-makers and investors in the development and implementation of climate-friendly measures in the transport sector. Info: klimaaktivmobil.at