Within the transportation sector, the United States has the highest rate of greenhouse-gas emissions. In 2008, U.S. vehicle emissions accounted for 33% of nationwide carbon-dioxide emissions, with half of that coming from urban passenger transport, according to the report.

Due to a high volume of emissions, the U.S. transportation sector has been targeted for emission-reduction strategies. These strategies begin with a baseline study to establish the level of pollution and the contributing factors. Emission studies conducted by cities, regions, and states are the first step in developing a climate action plan.

"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," the report states.

To conduct a greenhouse-gas emissions inventory, researchers must establish the scale at which the emissions will be measured, the method for measuring emissions, and the timeframe of the study. Researchers must decide where to draw the boundary between emissions that are associated with the transportation sector, and those that are not. They must also establish geographical boundaries, and decide how to count trips that might cross city or state lines.

"Depending on the method chosen for the study, the results can be significantly different," the report states. "The chosen methodology can have a large impact on the set of strategies chosen by a city or a region in developing a comprehensive climate action plan or undertaking stand-alone transportation greenhouse-gas emissions mitigation actions."

The 19-page report, "Citywide Transportation Greenhouse Gas Emissions Inventories: A Review of Selected Methodologies," is available at bit.ly/GHGReport102910.