The general approach to quantifying the emissions-related benefits or disbenefits of a transportation project involves the following steps:
Predict how a particular project will affect travel activity. Travel activity may include number and type of vehicles, diversion of traffic to other routes or modes, and also the operating speeds and other operation characteristics of traffic flow.
Estimate the changes in the amount, type and location of air pollution emitted. This step requires using a vehicle emission model to determine emission rates and/or emission levels for the the base case and project alternative.
Monetize (measure emissions in monetary units) emission costs by applying the cost of emissions. The cost of emissions are usually measured in dollars per ton (or metric ton) or cents per kilogram.
Vehicle emission models, such as the EPA's MOVES model, California's EMFAC model and other similar mobile source emissions models can be used to predict the per-mile vehicle emissions under various vehicle operating conditions and location specific atmospheric conditions. The following factors tend to affect emission rates:
Vehicle type. Larger vehicles tend to produce more emissions.
Vehicle age and condition. Older vehicles have less effective emission control systems. Vehicles with faulty emission control systems have high emissions.
Driving cycle. Emission rates tend to be relatively high when engines are cold.
Driving style. Faster accelerations tend to increase emission rates.
Operating conditions. Emissions per mile increase under hilly and stop-and-go conditions, and at low and high speeds. As a result, energy consumption and emissions are likely to decline if roadway conditions shift from Level of Service (LOS) F to D, but are likely to increase with shifts from LOS D to A.
Because vehicle fleet mix and technologies change over time, it is important to use the most current and accurate available modeling software and fleet data when evaluating transportation emissions. The major climate change emissions (particularly carbon dioxide) are closely related to fossil fuel consumption, so vehicle fuel consumption rates are a good indicator of overall climate change emissions.
The table below summarizes estimated costs per metric tonne of emissions, and unit costs per vehicle-mile, based on various studies. Some jurisdictions or government agencies have standard units to be used when evaluating air pollution costs.
Regional Pollution Studies Summary Table – Selected Studies (Litman 2009)
Issues when Estimating Emissions Benefits
There are several issues that may arise when estimating emissions benefits. Assumptions and sensitivities in the results due to these issues should be noted in the presentation of the results:
Analysis varies in scope. Some emission models and cost estimates only consider a limited set of emissions and impacts, such as just ozone and carbon monoxide, but not particulates and climate change emissions, or just human health impacts and not agricultural and ecological damages.
Models are not very accurate at predicting emission and exposure impacts. Current models are not very accurate at predicting how factors such as changes in driving cycles and congestion impacts will affect overall emissions, nor how human health is affected by exactly where people are exposed, and who is affected. For example, there is currently poor information on the health risks to motorists of driving on busy roadways, nor the relative risk of children compared with adults.
There is much that is not understood about the relationship between emissions and human health. For example, it is unclear whether the combined effect of several pollutants is worse than the effect of a single pollutant. Other pollutants may only affect human health once their ambient concentration is above a certain threshold.
A major threat to human and plant health is ozone, which is created from the combination of nitrogen oxides and hydrocarbons. In any particular situation, one of these gases will be the limiting factor on how much ozone is produced. Changes in emissions of this gas will change the amount of ozone created. Changes in emissions of the other gas will have no effect on the amount of ozone.
Current estimation methods are not good at predicting emissions "hot spots," where people are exposed to an exceptionally large amount of emissions. For example, high-rise buildings can change the way that emissions are dispersed, and bicyclists riding in heavy traffic may inhale more emissions than most other people.
AEA Technology (2005), Damages Per Tonne Emission of PM2.5, NH3, SO2, NOx and VOCs From Each EU25 Member State, Clean Air for Europe Programme, European Commission (http://ec.europa.eu/index_en.htm).
Booz-Allen & Hamilton Inc. California Life-Cycle Benefit/Cost Analysis Model (Cal-B/C) Technical Supplement to User's Guide. California Department of Transportation (Caltrans), 1999. Available at: http://www.dot.ca.gov/hq/tpp/tools_files/tech_supp.pdf.
Mikhail Chester and Arpad Horvath (2008), Environmental Life-cycle Assessment of Passenger Transportation: A Detailed Methodology for Energy, Greenhouse Gas and Criteria Pollutant Inventories of Automobiles, Buses, Light Rail, Heavy Rail and Air v.2, Paper vwp-2008-2, UC Berkeley Center for Future Urban Transport (www.its.berkeley.edu/volvocenter). Available at: www.sustainable-transportation.com.
Mark A. Delucchi (2005) A Multi-Country Analysis of Lifecycle Emissions from Transportation Fuels and Motor Vehicles, Institute of Transportation Studies, University of California Davis (www.its.ucdavis.edu). Available at:www.its.ucdavis.edu/publications/2005/UCD-ITS-RR-05-10.pdf.
DfT (2009), Transport Analysis Guidance: 3.3.5: The Greenhouse Gases Sub-Objective, Department for Transport (www.dft.gov.uk). Available at: www.dft.gov.uk/webtag/documents/expert/unit3.3.5.php.
EC (2005), ExternE: Externalities of Energy - Methodology 2005 Update, Directorate-General for Research Sustainable Energy Systems, European Commission (www.externe.info). Available at: www.externe.info/brussels/methup05a.pdf.
EDRG (2007), Monetary Valuation of Hard-to-Quantify Transportation Impacts: Valuing Environmental, Health/Safety & Economic Development Impacts, NCHRP 8-36-61, National Cooperative Highway Research Program (www.trb.org/nchrp). Available at: www.statewideplanning.org/_resources/63_NCHRP8-36-61.pdf.
EEA (2008), Climate For a Transport Change, European Environmental Agency (www.eea.europa.eu). Available at: http://reports.eea.europa.eu/eea_report_2008_1/en/EEA_report_1_2008_TERM.PDF.
Environmental Valuation Reference Inventory (www.evri.ca) is a searchable storehouse of empirical studies on the economic value of environmental benefits and human health effects.
Federal Register (U.S.). (2010). “Environmental Protection Agency and Department of Transportation, National Highway Traffic Safety Administration: Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards; Final Rule.” Vol. 75, No. 88 (May 7, 2010).
ITDP and CAI-Asia Center (2010), Transport Emissions Evaluation Models for Projects (TEEMP), Clean Air Initiative for Asian Cities (www.cleanairinitiative.org) and the Institute for Transportation and Development Policy (www.itdp.org); at www.cleanairinitiative.org/portal/node/6941. These Excel-based TEEMP models were developed for evaluating the emissions impacts of Asian Development Bank’s transport projects (www.adb.org/Documents/Evaluation/Knowledge-Briefs/REG/EKB-REG-2010-16/default.asp) and were modified and extended by ITDP, CAI-Asia and Cambridge Systematics for the for Global Environmental Facility (www.thegef.org) Scientific and Technical Advisory Panel (STAP). The Manual for Calculating Greenhouse Gas Benefits of Global Environmental Facility Transportation Projects (www.thegef.org/gef/GEF_C39_Inf.16_Manual_Greenhouse_Gas_Benefits) provide step-by-step instructions for developing baseline and impact estimations for various types of transport policies and projects, including transport efficiency improvement, public transport, non-motorized transport, transport demand management, and comprehensive transport strategies.
Todd Litman (2009), “Evaluating Carbon Taxes As An Energy Conservation And Emission Reduction Strategy,” Transportation Research Record 2139, Transportation Research Board (www.trb.org), pp. 125-132; based on Carbon Taxes: Tax What You Burn, Not What You Earn, Victoria Transport Policy Institute (www.vtpi.org). Available at:www.vtpi.org/carbontax.pdf.
Todd Litman (2010), "Safety and Health Impacts," Transportation Cost and Benefit Analysis, Victoria Transport Policy Institute (www.vtpi.org). Available at www.vtpi.org/tca/tca0503.pdf.
M. Maibach, et al. (2008), Handbook on Estimation of External Cost in the Transport Sector, CE Delft (www.ce.nl). Available at: http://ec.europa.eu/transport/sustainable/doc/2008_costs_handbook.pdf.
Lena Nerhagen, Bertil Forsberg, Christer Johansson and Boel Lövenheim (2005), The External Costs of Traffic Air Pollution, Report 517, Swedish National Road and Transport Institute (www.vti.se).
Nadine Unger, et al. (2011), “Attribution Of Climate Forcing To Economic Sectors,” Proceedings of the National Academy of Sciences of the U.S. (www.pnas.org): at www.pnas.org/content/early/2010/02/02/0906548107.abstract.
Urban Transportation Emissions Calculator (www.tc.gc.ca/UTEC) provides tools for estimating greenhouse gas (GHG) and criteria air pollution emissions from various types of vehicles.
US Environmental Protection Agency (EPA). (2011). MOVES (Motor Vehicle Emissions Simulator) Model. Available at: http://www.epa.gov/otaq/models/moves/index.htm.
Anming Zhang, Anthony E. Boardman, David Gillen and W.G. Waters II (2005), Towards Estimating the Social and Environmental Costs of Transportation in Canada, Centre for Transportation Studies, University of British Columbia (www.sauder.ubc.ca/cts), for Transport Canada. Available at: www.sauder.ubc.ca/cts/docs/Full-TC-report-Updated-November05.pdf.