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Emissions from transport are a key issue in urban areas particularly where there are high volumes of vehicles and/or significant levels of congestion. Transport interventions are implemented to try and improve traffic flows and/or reduce traffic volumes. To assess the impacts of transport interventions on emissions one must undertake an emissions assessment. These can have a number of functions: a) Estimate the likely future impacts of a measure or strategy; b) Assist planners and policy makers to design measures or strategies to reduce emissions; c) Quantify actual impacts of already implemented measures/strategies.

This Centro de Consulta – Reducción de Emisiones is designed to provide basic guidance and recommendations as to the tools and models available to assess GHG and local pollutant emissions from urban transport interventions in Latin America and the Caribbean Region. A database is also provided for factsheets and more detailed information. The full critical review which underpins the Centro de Consulta – Reducción de Emisiones recommendations can be found in the Centro de Consulta – Reducción de Emisiones database.

1. Calculating emissions

In theory, estimating GHG and air pollutant emissions from the transport sector should be relatively straightforward, because emissions are related to the amount and type of fuel consumed by the sector and the activity of the fleet.  In its most simple format emissions can be calculated as vehicle activity multiplied by the emission factors (i.e. the amount of emissions per vehicle).  This can be expressed most simply as:

E = A x Fe

E = Total Emissions

Fe = Emission Factor (amount of a certain pollutant that is produced per unit of distance travelled (usually gr/km))

A = Vehicle Activity (kilometers travelled and amount of vehicles in the fleet)

In practice, however, such calculations involve a number of variables, because the emission factor depends on the amount and type of fuel consumed and this in turn is dependent upon a wide range of inter-related factors, including the composition, technology and conditions of the vehicle fleet; the types and specifications of fuels available; the use and activity of the vehicle fleet; mode choice decisions; land use policies; economic conditions; and many other factors.  Often, data related to these factors is limited, is of questionable quality, or does not exist, and there are significant costs associated with collecting such data.

Estimating emissions from transport and the changes in emissions due to changes in any of the variables presented above is not an easy task.  The challenge is particularly complex when attempting to agree on forecasted (future) emissions, such as in establishing baselines or in estimating the potential impact of projects and programs. This is because assumptions need to be made about a variety of factors, including future economic conditions, vehicle fleets, and mode choice decisions, and these assumptions create uncertainty.  The figure below demonstrates the key emissions calculations required to robustly assess a measure or strategy.  This demonstrates the need to calculate total emissions under three conditions: the base year; without any intervention (business as usual or BAU); and with the transport intervention.

The methods used to estimate total emissions at each of the three points varies depending upon when in time the assessment is being undertaken.  An assessment can be undertaken prior to the implementation of a project ( ex-ante   ex-ante
An assessment undertaken before implementation is termed ex-ante. This type of study is undertaken in the base year and makes projections as to the likely emissions in a future year “with” and “without” intervention. The base year calculation for total emissions (E) can therefore be undertaken with known, or observed, emission factor (Fe) and activity (A) data, but both of the future scenarios, “with” and “without” the intervention, require future projections for Fe and A to allow the overall change in emission with the intervention to be estimated.
), or can be undertaken after implementation (ex-post   ex-post
An assessment undertaken after implementation is termed ex-post. This type of study is undertaken in a chosen year after completion of the intervention project. In this instance the known variables are the emissions factors (Fe) and the fleet activity (A) for the “with” intervention scenario. To estimate the potential change in emissions as a result of the project it is necessary to estimate total emissions for the “without” business as usual scenario as well as for the base year. In some cases the base year data may be available but in others it may be necessary to project backwards to estimate Fe and A for the base year.

The complexity of the assessment task described above, coupled with large variations in data availability and other local conditions, has helped lead toward the development of a variety of tools for estimating emissions from the transport sector.  This Consultation Center - Reducing Emissions  presents an Assessment Framework to assist with the development and analysis of urban transport projects and provides details of methods, models and tools available for the emission estimation of greenhouse gases and air pollutants are most suited to transport intervention projects in the LAC region.

Critical Review Summary »
Centro de Consulta – Reducción de Emisiones Database
See Factsheets for information on the tools, methods and methodologies discussed within the Centro de Consulta – Reducción de Emisiones and Critical Review, and links to further information.