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.
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
The average number of hot days has increased significantly in recent decades.
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 driving is still released into the environment when the car is parked.
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
Large green spaces cool the environment
Green infrastructure such as street trees, green roofs and walls, parks and gardens help mitigate the urban heat island effect. Green and blue spaces reduce the surface temperatures by up to eight degrees Celsius compared to built-up areas. Grassland has the least impact, while water areas have the greatest impact. Increasing the share of green spaces by ten percentage points lowers the average surface temperature by 0.4 degrees Celsius. Large green spaces, such as parks, reduce the temperature of the surrounding built environment up to a distance of two to three kilometres.27 Studies suggest that a proportion of about 40 per cent of green space in urban areas can reduce heat stress in people by as much as half. The circulation of air provided by large green spaces also has a cooling effect.28 Urban planning should therefore analyse and take into account existing urban cold-air corridors and should not obstruct them under any circumstances.
Reducing the heat island effect caused by traffic
Paved road surfaces, together with the waste heat generated by cars, contribute significantly to the formation of urban heat islands. To counteract this, we need fewer roads and more green spaces and water bodies that have a cooling effect. We need an attractive and shaded infrastructure for more active mobility and also more public transport because energy-efficient mobility instead of car traffic also produces less waste heat. Reducing surface sealing and car traffic not only mitigates urban heat stress, but also improves air quality and thus the quality of life and health of city residents.
How street trees cool down a city
Medellin, Colombia's second largest city, underwent a major redesign and greening programme in 2016 to combat increasing heat exposure, which led to the creation of 30 green corridors, including 20 kilometres of shaded cycle and walking paths. Along these corridors, 880,000 trees and 2.5 million plants were planted. In total, 65 hectares of new green space were created where the average temperature is 4.5 degrees Celsius lower. The effect of the greenery can be felt all over the city. Medellin's temperature already fell by around two degrees Celsius in the first three years of the project.29
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.
VCÖ recommendations
Use transport as a lever for cooler cities and promote green spaces
- Increase the number of unsealed and shaded parking spaces; park cars in garages so that more areas can be unsealed.
- Rightsize streets, use light-coloured materials for road construction and check potential unsealing measures for each planned construction site.
- Establish 'street parks', limiting the amount of space available for car traffic and requiring a minimum percentage of shading and unsealing.
- Identify unsealing potentials in cities and implement greening measures.
- Implement high-quality green spaces, applying the sponge city principle and promote green space monitoring.
Promote energy-efficient mobility
- Provide shade for walking and cycling infrastructure and public transport stops and stations.
- Promote the use of small and efficient electric vehicles.
- Implement more traffic-calming measures in towns and cities.
- Introduce lower speed limits on hot days.
- Increase the number of days employees can work from home on hot days.