This fall, the Transportation Planning Board will forecast vehicle-related emissions of ozone-forming pollutants in the Washington region for the year 2015. The forecasts are required under a new, more stringent federal standard for ground-level ozone concentrations that was first proposed in 2008 by the U.S. Environmental Protection Agency and finalized in May of this year.
The emissions forecasts are intended to determine whether changes in travel patterns through 2015 -- which can be affected in part by what improvements are or are not made to the region's transportation system -- will lead emissions of the two key ingredients in the formation of ground-level ozone -- volatile organic compounds, or VOCs, and nitrogen oxides, or NOx -- to exceed budgets for future emissions set in 2007 under the previous, less stringent standard.
The TPB is using the 2007 budgets because EPA requires metropolitan areas to use the budgets that they have most recently adopted, even if they were adopted under an older standard.
And, under the new standard, EPA designated the Washington region as a "marginal non-attainment" area, which means that it does not currently meet the new standard but is, based on observed trends, on track to do so by a deadline of December 31, 2015. Regions that are designated as "marginal non-attainment" areas are not required to set new budgets but must still meet the new standard by the 2015 deadline.
Earlier this year, the TPB generated forecasts of vehicle-related VOC and NOx emissions out to 2040, and for intermediate years of 2017, 2020, and 2030, because Federal Clean Air Act regulations require metropolitan areas to forecast future emissions periodically to reflect updates that have been made to short- or long-term transportation plans.
The TPB's analysis of the 2012 updates to the region's constrained long-range transportation plan, or CLRP, and the six-year transportation improvement program, or TIP, found that emissions of VOCs and NOx in 2040, as well as in the intermediate years, are expected to remain below approved levels. A forecast for 2015, the deadline for meeting the new EPA ozone standards, was not included in the assessment, which is why the new one is needed.
Analysts rely on two key kinds of information to forecast future vehicle-related emissions: projections of future population and employment growth and where that growth will occur; and predictions of where, when, and how people will travel in the future.
Population and employment projections are released periodically by the Metropolitan Washington Council of Governments. COG compiles the projections, known as the Cooperative Forecasts, using estimates made jointly by the county and municipal governments in the region. The most recent update to the Cooperative Forecasts -- Round 8.1 -- was approved by the COG Board of Directors in July.
Knowing where future growth is expected to occur, transportation engineers at the TPB then use a detailed computer model to predict future travel patterns based on the transportation projects and programs that are currently expected to be built or implemented in the short- and long-term.
The in-house model they use, known as the Version 2.3 Travel Demand Model, relies on the distribution of population and employment and what roads and transit lines will exist in the future, among other things, to predict where and when people will travel, and what travel mode they'll choose to get there.
The model predicts numerous outcomes related to the transportation system, but some of the key ones are the total number of trips made by private automobile or by transit on a typical day and the total number of vehicle-miles of travel, or VMT, in the region.
Analysts use these numbers along with detailed data about the ages and models of all the vehicles registered in the region to estimate total emissions of VOCs, NOx, fine particle pollution, and carbon monoxide -- the four vehicle-related emissions regulated by EPA -- using a computer model known as Mobile 6.2.
For more than a decade, vehicle-related emissions of VOCs and NOx have declined sharply, and are expected to continue declining through 2030. The TPB's emissions forecasts that are scheduled for this fall are expected to demonstrate that vehicle-related emissions will not jeopardize the region's ability to meet its own targets for emissions of ozone-forming pollutants over the next three years.