Researchers – led by Dr Haneen Khreis at the MRC Epidemiology Unit at the University of Cambridge – have created an interactive systematic evidence map of urban policy interventions to reduce traffic-related emissions and air pollution. Researchers, practitioners and policymakers can use this resource to discover more about the effectiveness of over 1000 policy scenarios, and help plan interventions in their own regions and cities.

Cities across the globe are hotspots for human exposure to air pollution, which in many regions comes largely from traffic. As urban populations continue to grow, more and more people are being exposed to traffic-related air pollution and its severe health effects.

In many cities, there is therefore both a need and possibility for improvement in air quality through targeted policy interventions. Researchers have developed a systematic evidence map which catalogues peer-reviewed evidence on urban-level policy interventions aimed at reducing vehicle emissions and traffic-related pollution. They also documented a range of health, climate, social and economic benefits.

• The team have built an interactive visualisation tool to explore all these interventions: https://tableau.tamu.edu/t/TTI/views/SEMDataVisualizationV2/SEMVisualizationDashboard
• The evidence recorded for each intervention is also hosted in an open-access, queryable Excel database available on the CARTEEH Data Hub: https://carteehdata.org/library/dataset/urban-policy-intervention-f08c

The systematic evidence map includes 376 unique articles, including 58 unique policy interventions, and 1,139 unique policy scenarios (a policy scenario is a single policy intervention, a variation of that same policy intervention, or two or more interventions bundled in the same policy package which was studied as a whole). Researcher examined scenarios across six policy categories:

• Pricing (e.g. congestion charging, road pricing, parking charges)
• Land-use (e.g. urban transport planning, housing planning)
• Infrastructure (e.g. mass transit development, street ventilation, vegetative roadside barriers)
• Behaviour (e.g. flexible work arrangements, rideshare schemes, active travel promotion)
• Technology (e.g. electric vehicles, speed control technology, real-time passenger information)
• Management, Standards and Services (e.g. Fleet management, low emissions zones, speed limits.)

The most frequently studied intervention was the use of alternative fuel technology, such as electric and hybrid vehicles, natural gas, hydrogen fuels etc. with the least studied interventions being vehicle ownership taxes, and studded tire regulations, each only studied once.  A mere 3% of studies addressed all elements necessary to effectively address traffic pollution and its public health impact – the so called ‘full-chain’ of interventions required between emissions, pollution, exposures, and health impacts.

Lead researcher Dr Haneen Khreis said:

The way that studies and interventions are currently weighted around certain areas, as well as the gaps in the evidence, demonstrate how the transport sector is mostly regulated based on traffic emissions. Much less attention is given to the knock-on effects on human health. Holistic strategies that might combine technology with behaviour and land use measures impacting demand are often neglected in favour of supply-side measures catering for an existing and sometimes increasing demand.

There are currently also gaps in regional coverage of studies. Areas that are most in need of interventions – rapidly urbanising low- and middle-income countries (LMICs) – are neglected in terms of where studies are currently happening. We hope these resources can help researchers and policymakers redress some of these imbalances and we are already working to expand health impact assessment models of air pollution to LMICs.”

Using these Open Access tools, researchers, practitioners, policymakers and third sector organisations can explore the effectiveness of different interventions and policy scenarios, and help plan for interventions in their urban areas – including options that they may not have been previously aware off. For example, an intervention not currently considered in one country may be well documented in another.

Users can also dig deeper into the impact of these interventions on both primary outcomes (traffic emissions and traffic-related air pollution), secondary outcomes (human exposure and health impacts) and other documented aspects of interest (for example, what co-benefits might be expected when a particular policy intervention is implemented? What enablers and barriers may impact transferability potential?). Information on whether certain policies were documented to reduce, increase, or have no impact on twenty-five different air pollutants also form part of the created tools. The most frequently documented pollutants were nitrogen oxides, carbon monoxide, fine and coarse particulate matter, and hydrocarbons.

Researchers, meanwhile can use these resources to explore the current state of the evidence and devise future research questions. For example, they can see which interventions and outcomes have been studied most frequently and decide if there is enough evidence available to conduct a systematic review or a meta-analysis on the effectiveness of a particular intervention. Or look for gaps in the literature which may impact practice, for example whether data is lacking for a certain intervention or outcome.

This is the first peer-reviewed systematic evidence map to compile international evidence on urban-level policy interventions to reduce traffic emissions the context of human exposure and health effects, whilst also recording enablers, barriers, and co-benefits.

Further information

• For more information contact: Dr Haneen Khreis, Senior Research Associate, MRC Epidemiology Unit hrk38@medschl.cam.ac.uk
• Full paper:  Khreis, H., Sanchez, K.A., Foster, M., Burns, J., Nieuwenhuijsen, M.J., Jaikumar, R., Ramani, T. and Zietsman, J., 2023. Urban Policy Interventions to Reduce Traffic-Related Emissions and Air Pollution: A Systematic Evidence Map. Environment International, p.107805. https://doi.org/10.1016/j.envint.2023.107805
• Open access Excel database with catalogued studies and policy scenarios:  https://carteehdata.org/library/dataset/urban-policy-intervention-f08c
• Open access interactive visualization tool: https://tableau.tamu.edu/t/TTI/views/SEMDataVisualizationV2/SEMVisualizationDashboard?%3Aembed=y&%3AisGuestRedirectFromVizportal=y
• This work was conducted with support from the Texas A&M Transportation Institute’s Center for Advancing Research in Transportation Emissions, Energy, and Health, a U.S. Department of Transportation’s University Transportation Center, College Station, TX (grant number: 69A3551747128) and a European Research Council (ERC) grant under the Horizon 2020 research and innovation programme (gran number: 817754)
• The work adheres to a protocol which has been published a priori: Sanchez, K.A., Foster, M., Nieuwenhuijsen, M.J., May, A.D., Ramani, T., Zietsman, J. and Khreis, H., 2020. Urban policy interventions to reduce traffic emissions and traffic-related air pollution: Protocol for a systematic evidence map. Environment international, 142, p.105826. https://doi.org/10.1016/j.envint.2020.105826