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Integrating Climate Change into the Transportation Planning Process

Chapter 5: Quantification of GHG Impacts in Transportation Plans

Quantifying GHG emissions from transportation plans is a new area and one fraught with uncertainty. There are limitations in the ability of existing models to estimate the emissions generated by current and forecast transportation systems. Existing models may not adequately capture the potential reductions in emissions from certain strategies. While many agencies recognize that they will eventually need to estimate GHG emissions from plans and strategies, there are no standard tools or approaches yet.

Despite these uncertainties, DOTs and MPOs are taking some steps to quantify transportation GHG emissions now. Table 4 summarizes our findings on quantification requirements and processes among states, state DOTs, and MPOs. A number of agencies are using best available methods to quantify GHG emissions. Others are developing new methods and tools. The robustness and technical complexity of methods employed varies substantially. In some states, agencies are required–or will probably soon be required–to quantify GHG emissions. For those agencies instituting performance measures or targets related to GHG emissions, quantification is an indispensable step in the planning process.

Table 4: State and MPO Quantification of GHG Emissions from Transportation Plans
State requirement for quantification Current
  • New York
  • California
  • Oregon
  • Washington
DOT and MPO quantification Completed
  • Albany, NY MPO
  • Sacramento MPO
  • San Diego MPO
  • Southern California MPO
  • Maine DOT
  • Chicago MPO
  • Missoula MPO
  • Philadelphia MPO
  • Portland MPO
  • San Francisco MPO
  • Seattle MPO

* These states have proposed requirements in official documents.

For those DOTs and MPOs pursuing quantification of GHG emissions, quantification can include one or more of several elements. Transportation GHG inventories document historic (and possibly projected) GHG emissions. Inventories are often created as part of statewide climate action plans. Climate action plans often quantify changes in emissions from mitigation strategies as well. Some metropolitan regions are now also creating GHG inventories and climate action plans. Within the transportation planning process, emissions resulting from LRTP alternative investment packages can be calculated. Emissions resulting from specific projects or bundles of projects can also be calculated.

5.1. Transportation Plan GHG Impacts - Current Practice

Many transportation agencies are not currently attempting to quantify GHG emissions from transportation plans. Those agencies that are working to quantify emissions are taking different approaches, commensurate with their levels of resources and expertise in the area.

Metropolitan Transportation Commission

The Metropolitan Transportation Commission (MTC), the MPO for the San Francisco Bay Area, will adopt performance targets for GHG emissions in the next version of its RTP, a draft of which is due for release in December 2008. The preliminary target is to reduce CO2 emissions 40% below 1990 levels by 2035. A second preliminary target is to reduce VMT per capita by 10 percent by 2035. The planning process has established these targets upfront. Proposed packages of investments are being analyzed for their ability to meet these and other targets. Packages include freeway investment with modest efficiency improvements, a high-occupancy toll (HOT) network with expanded express bus service, an expansion of rail transit, a comprehensive road-pricing policy, and a land-use strategy based on smart growth principles. The approach to analysis differs from past planning efforts, in which system performance was assessed only after packages of investments were proposed. The plan's preliminary CO2 targets are based on California's state-level mandates, including AB32.1

MTC is currently evaluating the CO2 impacts of individual highway and transit projects. This analysis will feed into a performance comparison of projects. A project-level GHG analysis raises some key issues such as where to set the boundaries for analysis and how to account for potential mode shift.

Puget Sound Regional Council

The Puget Sound Regional Council, the MPO for the Seattle area, is using the U.S. EPA's Motor Vehicle Emission Simulator (MOVES) model to do a regional level analysis of GHG emissions in its LRTP. As part of the analysis, PSRC is helping to pilot an updated version of MOVES that includes improved sensitivity of emissions to vehicle speeds. PSRC also has received a grant from FHWA to make improvements in their travel demand model. The improvements will capture the impacts of factors like mode choice and the cost of driving. Outputs from the model will feed into the GHG emissions analysis.

Missoula County

Missoula County serves as the MPO for Missoula, Montana and is currently updating its LRTP. The update process includes a regional land use and transportation visioning exercise called Envision Missoula. Missoula plans to provide a basic estimate of CO2 emissions from the plan, probably using a simple VMT multiplier applied to the outputs of the travel demand model. The agency would be open to receiving guidance on other appropriate techniques for GHG emission quantification.

Capital District Transportation Committee

Albany's Capital District Transportation Committee (CDTC) incorporates analysis of GHG emissions in its planning process in two ways. First, CDTC applies a "full cost analysis”, including analysis of global warming costs, to major system decisions. Full cost analysis is also used to evaluate candidate TIP projects, when applicable. Second, in compliance with New York State requirements, the agency estimates the GHG emissions resulting from its LRTP. (As described in Section 3.2, New York requires MPOs to estimate the energy and CO2 emissions from their long range transportation plans and also from their transportation improvement programs.) CDTC modified is post-processor to produce GHG emissions specific to year, functional classification, and operating speed.

CDTC has taken an innovative approach to the use of their travel demand modeling. The region has been proactive in encouraging concentrated, sustainable development patterns, and has a focused interest in establishing linkages between policy setting and environmental responsibility. CDTC forecast a 15% reduction in trip generation per household based on a range of policies and principles, such as urban reinvestment, transit oriented development, and bus rapid transit. CDTC believes that travel demand forecasts are partly a self-fulfilling prophecy, since forecasts are used in project development and design. They are advocating no new major highway construction, and that any widening involve managed lanes and HOV. Based on these assumptions, CDTC has calculated impacts on GHG emissions, fuel consumption, and air quality emissions. The New York State Department of Environmental Conservation (NYSDEC), which reviews CDTC's air quality and conformity reports, has not accepted the growth forecast, saying it understates future problems. Both CDTC and NYSDOT are currently advocating for the approach.

Sacramento Area Council of Governments

The Sacramento Area Council of Governments (SACOG) used the results of a report from the California Energy Commission (CEC) to estimate the impact of its LRTP on CO2 emissions. The report, entitled "Effect of Land Use Choices on Transportation Fuel Demand”, estimated the fuel savings associated with SACOG's LRTP. In addition, SACOG is working with several modeling applications to analyze the impacts of different transportation and land use scenarios. SacSim, the agency's new travel demand forecasting model, is the first regional model to use individual land parcels as the level of input data.2 The agency is working to create new linkages between its travel demand model and models that include land use and economic forecasts. SACOG is also working to improve its model of commercial vehicle travel behavior.

SACOG has estimated that its share of the statewide GHG reduction goal for the regional transportation/smart growth sector is 1 million metric tons CO2-equivalent (MMtCO2e).3 The EIR used this share to determine the impact significance of the LRTP on climate change.

5.2. Transportation GHG Inventories - Current Practice

A majority of states have developed statewide GHG inventories as a reporting tool to track annual emissions and inform policy development. Statewide GHG inventories typically provide detailed information on the volumes, types, and sources of emissions and allow for comparisons of emissions over time and across source types.

Some MPOs are also involved in regional GHG estimation efforts as part of a broader regional climate change planning process. Development of a regional GHG inventory is a relatively new concept; two regions, the Philadelphia and Washington, DC urban areas, are currently engaged in this process with support from EPA. Transportation emissions are one component of GHG inventories.

Delaware Valley Regional Planning Commission, Philadelphia PA

The Delaware Valley Regional Planning Commission (DVRPC) is in the process of preparing a regional GHG inventory for 2005, as well as projected GHG emissions for 2035. Among the sources to be included in this inventory are emissions from on-road transportation, which are expected to be one of the region's primary sources of GHG emissions. CO2, CH4, and N2O emissions will be calculated using modeled estimates of annual average daily vehicle miles traveled (VMT) by vehicle type and road class. Per mile emissions factors will be applied to the VMT totals.

The VMT estimates will be generated by DVRPC's regional transportation model, which is used to support the region's long range transportation planning and air quality conformity analysis process. The regional transportation model will also be used to generate GHG emissions estimates for various transportation plan alternatives. For both the 2005 estimates and 2035 projections, the VMT will be reported for each county. Thus the region's transportation emissions can be allocated to the county level. Preliminary estimates are expected from this effort by summer 2008, with final results by November 2008.

Metropolitan Washington Council of Governments, Washington DC

A regional inventory of CO2 emissions from transportation was developed by the Metropolitan Washington Council of Governments (MWCOG) while calculating 1990 and 2005 emissions of criteria pollutants for air quality conformity purposes. CO2 estimates from mobile sources were calculated using data and forecasts of vehicle miles of travel (VMT) by vehicle type from the air quality conformity analysis. Emission factors were modeled using MOBILE6 and travel patterns in the COG region on network and local roadways. The inventoried emissions included auto access to transit and diesel transit and school buses. Emissions forecasts to 2030 were developed using assumptions in the MOBILE6 model (such as changes in fuel mix over time) and the COG's travel forecasting model.

MWCOG has proposed two GHG emission reduction scenarios for development. The first scenario reflects the current procedures of the LRTP and uses goals that are within reach fiscally and administratively, but that improve the conditions of the 2030 baseline. The second scenario examines how new long-term goals could be achieved using various combinations of interventions, including improved fuel efficiency, alternative fuels, and reducing vehicle travel. The first step in developing this scenario is identifying a CO2 emission reduction goal. MWCOG has noted that one of the main benefits from establishing a reduction goal is that many of the strategies used to reduce CO2 emissions will also provide ancillary transportation, environmental, health, and economic benefits, such as reduced congestion and fuel consumption. The COG Climate Change Steering Committee has discussed a proposed regional goal of reducing overall regional CO2 emissions by 70-80% below 2005 levels by 2050.

5.3. GHG Impacts from Construction - Current Practice

Most agencies are focusing on estimating the emissions associated with the use of transportation facilities. A few agencies are also attempting to quantify GHG emissions associated with construction and maintenance of transportation facilities.

Metropolitan Transportation Commission

At the request of the U.S. Environmental Protection Agency (EPA), MTC is considering how to quantify emissions from construction of transportation facilities. The agency plans to estimate CO2 emissions from construction in the Environmental Impact Report (EIR) for its RTP. No methodology has yet been determined. One possible approach is to use outputs from the Sacramento Air Quality Management District's Roadway Construction Emissions Model, along with CO2 emissions factors from another source, to calculate emissions.

Capital District Transportation Committee

Albany's CDTC quantified indirect energy use associated with their TIP. The NYSDOT methodology for calculating GHG emissions describes indirect energy as the energy required to construct and maintain transportation facilities. The CDTC estimate of this energy use was 931 billion Btu over the 5-year analysis period of the TIP. However, indirect impacts could not be estimated without a detailed and very time-consuming analysis of specific project schedules in each region. CDTC used NYSDOT's methodology to perform the calculations, and only included energy for road construction, and only addressed projects listed in the TIP, not additional projects proposed in the long range transportation plan.

5.4. Barriers and Needs

Quantification of GHG emissions is one of the most challenging aspects of integrating climate change into transportation planning. There is room for improvement across the board in inventory techniques and techniques for estimating the impact of policies and strategies. Guidance is particularly needed in this area.

Most state-level GHG inventories for transportation have some major limitations for transportation policy analysis because they are not presented in sufficient detail to assess emissions reduction strategies. Inventories of transportation GHG emissions are typically developed by fuel type based on fuel sales data, while strategy analysis requires estimate of emissions for individual modes, vehicle types, and geographic areas. In addition, statewide inventories do not report emissions at the regional level, and therefore are of limited use for MPO planning and strategy analysis.

Consistency of transportation data is a problem for GHG inventories. The data on fuel sales that are used to calculate statewide inventories do not always match with data on vehicle miles traveled (VMT). Both cross-border travel and poor information on state fleets and fuel economy contribute to this discrepancy. These issues are magnified at smaller geographical levels. In developing regional transportation GHG inventories, MPOs will rely heavily on local VMT estimates and perhaps information on local travel conditions. Such local inventories are very likely to be inconsistent with state-level inventories. If and when regions are required to meet certain VMT or transportation GHG reduction goals, state and regional inventories would provide conflicting bases for performance measurement. The development of reliable and consistent transportation GHG inventories at the regional scale is therefore important.

The appropriate level at which to quantify emissions in transportation plans remains an area of uncertainty for transportation agencies. Regional level analyses compare GHG emissions between broad packages of modal and development strategies. Many agencies argue that quantitative analyses are not useful at finer grains, because GHG emissions are essentially determined at the regional level. Other agencies are proceeding with analyses down to the project level.

Guidance on appropriate quantification techniques for various agencies and planning components is needed. There is a high potential for disagreement over appropriate quantification methods at present. The experience of Albany's CDTC illustrates. While the current atmosphere allows agencies to be innovative in their approaches to quantification, agencies with fewer resources to develop and test quantification methods are at a significant disadvantage.

The current emissions models are another barrier to quantification of GHG emissions. While EPA's MOBILE6 model can produce CO2 emissions estimates, the CO2 emission factors do not vary with vehicle speed or driving cycle. For this reason, MOBILE6 is inappropriate for any kind of detailed analysis of transportation plan or project-level emissions, which are likely to involve changes to congestion levels and speeds. EPA's new MOVES2004 model does have GHG emission factors that vary with speed and driving cycle. However, rather than using the MOVES2004 current interface, users must manually modify the MOVES database and create specific driving cycles in order to take advantage of this information. This presents a challenge to many users of the model, and also makes it cumbersome to analyze a large number of scenarios.

As state and federal agencies consider regulations and guidance that involves quantification of transportation GHG emissions, they should keep in mind particularly the needs of smaller MPOs. For example, Eugene's CLMPO, with just four full time staff members, is concerned about having expectations or standards for climate change planning set by the larger MPOs in the state. It is therefore important that small MPOs such as CLMPO receive explicit consideration in the development of state or federal planning requirements. For example, unfunded state mandates can be particularly burdensome for small MPOs that rely exclusively on federal funding. CLMPO is actively participating in discussions on climate change at the state level in order to prevent any oversight of the needs and constraints of small MPOs.

  1. 1 Droettboom, Ted and Christy Riviere, "Breaking with the Past.” Planning. May 2008. American Planning Association, Chicago, IL. 33-34.
  2. 2 SACOG, "Comments on Placer Vineyards Specific Plan, Second Partially Recirculated Revised Draft EIR.” May 15, 2007
  3. 3 The statewide total (18 MMtCO2e) is as determined by the Climate Action Team in the report "Climate Action Team Proposed Early Actions to Mitigate Climate Change in California, Draft for Public Review” (CalEPA, April 2007)
Updated: 12/23/2016
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