This working paper analyzes the main methodological issues involved in making an citywide transportation emissions inventory and explores how they can influence the inventory’s results.

Executive Summary

The transportation sector accounted for 14.3% of world greenhouse gas (GHG) emissions in 2005, behind electricity and heat (24.9%) and industry (14.7%), but ahead of agriculture, land use change, and waste. As one of the three highest emitting activity sectors, transportation is an important field to target for emissions reductions strategies. It is even more crucial in the United States, where its share of emissions is considerably higher than in the rest of the world. The US transportation sector accounted for over 33% of total nationwide CO2 emissions in 2008. Urban passenger transport in the US represents almost half of total transportation emissions, and around 15% of total CO2 emissions, according to the Environmental Protection Agency (EPA).

The ability to perform accurate transportation emissions inventories at multiple geographic levels and update them regularly is critical for identifying opportunities for emissions mitigations activities, as well as for measuring their progress over time. EMBARQ – The World Resources Institute Center for Sustainable Transport is engaged in this area in order to assist local and national governments around the world to reduce GHG emissions. Reductions in GHG emissions represent one of the key performance indicators across all EMBARQ projects, from the low emissions zone in Istanbul’s historic peninsula to Bus Rapid Transit (BRT) corridors across Latin America and Asia.

A citywide transportation emissions inventory is critical in order for local actors to understand the magnitude of transportation emissions and evaluate the relative contribution of different factors to overall emissions. Furthermore, an accurate inventory is an essential step in developing a comprehensive climate action plan, an effort that many cities, regions, and states are undertaking.

Developing an inventory involves making important decisions about which emissions from what trips to include, what the boundary should be, and what data collection method should be used. Depending on how it addresses these different issues, an inventory for the same city or metropolitan area can report significantly different results. In this paper, we analyze the main methodological issues involved in making an inventory and explore how they can influence the inventory’s results. There is no single way to address all these challenges successfully, and each city’s decision on how to develop a methodology for creating an inventory will depend on the local political and geographical context, as well as data availability.

In the second part of the paper, we review several methodologies currently used around the world to develop citywide transportation emissions inventories, including international methodologies such as the World Bank Citywide Methodology, the European Commission’s COPERT model, as well as inventories developed by local planning agencies in San Francisco and Lisbon. These inventory methodologies vary significantly in terms of scope, data requirements, and data collection methodology. They also illustrate the diversity of approaches currently used around the world to track urban transportation emissions.

Finally, we discuss how transportation GHG emissions inventories could be integrated with climate policies in the US and internationally, noting that inventories would be particularly useful in implementing performance-based transportation funding, where federal funding would prioritize funding for projects that reduce GHG emissions. In the developing world, inventories provide the information that can inform emissions-reduction strategies and, when repeated over time, can help monitor the effect of projects and policies aimed at reducing emissions. This can help cities plan for GHG emissions reductions and apply for transportation-related Nationally Appropriate Mitigation Action (NAMA) financing.

This report was made possible with funding from the Rockefeller Foundation.