Air pollution in traffic light intersections


When the traffic lights turn yellow, many motorists think that they can get over the intersection if they gas up a little more, but the extra acceleration affect the health of pedestrians, cyclists and residents close to the intersection.

Raise in air pollution when the light is yellow
Copenhagen Solutions Lab have in collaboration with Leapcraft and DTU amongst others completed an investigative project of the linkage between traffic and air pollution in traffic light intersections.
The results of the project show an increase in air pollution levels when the light signal shows yellow, which may be due to the acceleration of vehicles that have been holding for red or are trying to reach the traffic light before switching to red. The increase in levels are up to:

  • 9 percent for Nitrogen Dioxide (NO2)
  • 113 percent for Carbon Monoxide (CO)

Process and equipment
The results are based on measurements in two traffic lights in Vanløse where several data sources have contributed with information about the local conditions, which have subsequently been analysed to find the connection.
Data sources used are:

  • Air pollution sensor
  • Traffic signal regimes
  • Traffic radar
  • Average speeds
  • Local weather stations

Presentation at the ITS world congress
The entire project was presented by Project Manager Christian Gaarde Nielsen at the World Congress for Intelligent Transport Systems (ITS) 2018. The presentation marked the end of an experimental trial, initiated by the City of Copenhagen in November 2017, based on Copenhagen's increasing work with understanding the context of air quality.
If you may like, please read the technical report that was submitted to the ITS World Congress or have a look at the presentation.

Need for further investigations
Although the results confirm the assumptions the municipality had before the start of the project, further studies are still needed before it can be concluded whether this new knowledge can be used to improve local air quality. The reason being that air pollution situations in context of traffic can easily change based on driving patterns, weather conditions and other parameters.
Therefore, further studies are needed to inform whether it is possible to change local air pollution by changing the signal times in several traffic lights along a traffic corridor. If this can be demonstrated, it must also be examined how air pollution is best distributed in terms of spreading the traffic out more evenly or accumulate traffic at selected locations.