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Climate Change Mitigation Peer Exchanges: Comprehensive Report

3. GHG Analysis and Incorporating GHGs into the Planning Process

MPOs and State DOTs have begun to incorporate GHG analysis into their planning efforts. These analyses typically include development of GHG inventories, forecasts, or analyses of the impacts of particular transportation strategies or investments. This section highlights some of the primary themes observed across the three exchanges with regard to how State DOTs and MPOs are approaching GHG analysis and integrating GHG considerations into long-range planning.

State DOTs and MPOs have diverse approaches to GHG analysis

Because of differences in their planning processes and roles in the transportation system, State DOTs and MPOs tend to have different levels of experience and expertise in analyzing GHG emissions. In 2010, EPA released the Motor Vehicle Emissions Simulator (MOVES) model, which relies on VMT estimates to generate emissions estimates at the Sstate or county level and incorporates emissions rates based on vehicle type, year, speed, and other factors, allowing it to be more sensitive to changes in these variables. Many MPOs are transitioning from use of the MOBILE model to MOVES for conformity. However, while defaults are available, MOVES requires a large number of inputs, and generating MOVES inputs can be challenging, particularly for Sstates and regions without experience with conformity. In particular, many Sstates do not have statewide travel demand models and have limited modeling capacity to analyze GHG reduction strategies. While most MPOs have regional travel demand models, those in attainment often do not have staff experienced in air quality analysis.

GHG analysis at Sstate DOT and MPO levels has typically occurred during different types of planning processes as described below.

State DOTs: Development of Climate Action Plans

A number of Sstates have developed GHG inventories in order to determine their primary emissions sources as part of their development of a comprehensive climate action plan (CAP) covering all economic sectors. In many Sstates, the State's environment department led the CAP development process, and the level of DOT engagement has varied. Most of these inventories have quantified on-road GHG emissions based on fuel sales within the Sstate, and these inventories do not always match up well with estimates developed using travel data. Consequently, a number of State DOTs are looking at methodologies to more accurately estimate and forecast GHG emissions within their Sstates. For example:

MPOs: Analysis for the Long Range Planning Process

A number of MPOs have conducted GHG analyses as part of their long range transportation planning process, through development of GHG inventories and/or forecasting of GHG emissions over the time horizon of the plan under multiple scenarios. In some cases, MPOs also have conducted analyses of the potential impacts of strategies or initiatives such as investments in transit or support for biking or walking that are undertaken for the purpose of reducing emissions.

Of the MPOs participating in the peer exchanges, a significant proportion were experienced in both travel and emissions modeling, skills that transfer well to GHG analysis and that have allowed them to use EPA's MOVES model or other techniques for estimating GHG emissions based on regional VMT. Some lessons learned include the following:

This DVRPC figure shows greenhouse gas emissions per population and employment by municipality. A map breaks down the region into municipalities and highlights areas with high levels of GHGs in red and low levels of GHGs in green. Areas inbetween are various colors of orange and yellow. The central part of the map, Philadelphia, is all green, whereas the outer regions of the map are predominantly red.

Figure 1 . DVRPC Analysis of GHG Emissions Rates by Municipality

A GHG inventory can be presented in different ways, either assigned to the road network within a region or assigned to the locations that generate emissions.Regional emissions analysis conducted for criteria pollutants as part of the conformity process focuses on estimating the total emissions on the roadway network within a region. However, since GHG emissions are a global problem, not a localized air pollutant, there are different ways to look at transportation GHG emissions sources. Rather than simply calculate emissions in relation to vehicles on the road, some areas have attempted to attribute emissions to generators within the region. For example, in 2009, Delaware Valley Regional Planning Commission (DVRPC), which covers Philadelphia, completed an inventory for the calendar year 2005 using MOBILE6 (the precursor to MOVES). Because of the large number of municipalities in DVRPC's planning area, DVRPC has been working on allocating emissions and VMT to its various local governments as part of its inventory. Its methodology for this has been to allocate half of a trip's VMT to the origin and half to the destination but none to any pass-through area. This has allowed DVRPC to demonstrate the different amounts of VMT and GHG per capita associated with different types of land use and land use densities (See Figure 1).

A graph shows total emissions as 131.18 in 2005 on the far left side and total emissions in 2010 as 126.25 on the far right side. In the middle are various downward facing green arrows and upward facing red arrows that demonstrate what the decreases in emissions are attributed to and what the increases in emission are attributed to. The factors that contribute to emissions reduction and increase are both methodological and direct. For example, a changed natural gas emission factor resulted in a relative GHG increase between 2005 and 2010 (methodological), and a change in the demand for solid waste resulted in the decrease of GHG emissions between 2005 and 2010 (direct).

Figure 2. CMAP Explanation of Sector Emissions Changes 2005 - 2010

If updating emissions inventories over time, it is valuable to explain the reasons for changes in emissions, including methodological issues.As an example, the Chicago Metropolitan Agency for Planning (CMAP) has conducted comprehensive emissions inventories in 2000, 2005, and 2010. The inventories have shown relatively constant emissions levels across these years. However, changes in methodology over these years have altered the region's understanding of emission sources. For example, changes in demand for on-road transportation between 2005 and 2010 accounted for a decline in on-road transportation emissions, but this was overwhelmed by the increase in emissions found due to changes in methodology (See Figure 2). It is very valuable to help explain to the public and decisionmakers why emissions estimates have changed: because of actual changes in sources, or due to methodological changes.

In addition to completing inventories, a number of MPOs have analyzed the impact of various transportation emission reduction strategies or scenarios to see what ways might be most effective at reducing emissions. These analyses tend to incorporate a number of different methodologies including MOVES and off-model approaches. For example:

New tools are helping to overcome data and methodology challenges, though issues of uncertainty, compatibility, and comparability between methods remain.

Analyzing GHGs for transportation requires large amounts of data and often complex approaches - whether through models or "off-model" analysis. A number of new tools have been or are currently being developed to help States and MPOs consider both current emissions levels, opportunities to improve sustainability, and the impact of investment and policy choices. For example:

While new tools offer new possibilities for analysis, participants noted that rapid progress in the area of GHG accounting and modeling has meant that older analyses may no longer apply, causing a disconnect between previous understandings of a region's emissions profile and the current one. For example, while a number of States have performed emissions analysis based on fuel use, many MPOs have created VMT-based inventories. However, the results from these inventories performed using different methodologies often do not match, presenting a challenge for decisionmakers at both the State and MPO levels.

The data challenges illustrate that while improvements in modeling and inventory methodologies are helpful, there is still a high degree of uncertainty as models change and are updated.

Updated: 10/20/2015
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