TPB News

2014 CLRP Update Enters Next Phase: Air Quality Conformity Analysis

May 12, 2014

The 2014 update to the region’s Constrained Long-Range Transportation Plan has entered its next phase: the federally required Air Quality Conformity Analysis, to assess whether future vehicle emissions under the proposed plan will exceed regional budgets approved by the federal government. The analysis will be completed by September, in time for the Transportation Planning Board to review the results before considering the update for final approval in October.

The analysis takes place each year once the TPB and the public have had an opportunity to review and comment on the projects proposed to be included in the plan update, as well as the planning assumptions and technical methodologies that will be used to predict future emissions.

This year, the TPB voted at its April 16 meeting to approve the projects, including 10 major new ones, and all the other inputs to be used in the analysis of the 2014 CLRP update.


Key inputs in forecasting future vehicle emissions are forecasts from the Metropolitan Washington Council of Governments of how much population and job growth is expected in the region through 2040, and where that growth is expected to occur. The TPB uses this information in conjunction with the planned system of roadways and transit spelled out in the proposed CLRP to predict where, when, and how people will travel around the region in coming decades.

Those forecasts then form the basis for estimates of future vehicle emissions, taking into account how new vehicle technologies, like on-board emissions controls, more efficient engine systems, and cleaner-burning fuels, are expected to evolve, and how quickly those changes are expected to occur.

The federal Clean Air Act requires the air quality analysis as a way to help safeguard public health. The law specifically requires that observed concentrations of certain pollutants not exceed healthy levels, as determined by a team of scientists and doctors. Metropolitan areas with concentrations that exceed these standards must develop and adopt plans to meet the standards, including setting “budgets” for vehicle emissions of the pollutants. Metropolitan areas must periodically demonstrate, through air quality analysis of proposed transportation plans, that future emissions will not exceed those budgets.

In the Washington region, concentrations of ozone, which forms mainly in summer months from the combination of nitrogen oxides (NOx) and volatile organic compounds (VOCs), currently exceed federal standards. The region has adopted “budgets” of allowable emissions of NOx and VOCs and a plan to bring emissions below those levels.

Just recently, the region met the standard for fine particle pollution (PM2.5), a practically invisible soot-like compound that can cause respiratory ailments. Although the region now meets the standard, federal law requires that the region continue to forecast future PM2.5 emissions in order to demonstrate ongoing compliance with the federal standard. This is also the case with wintertime carbon monoxide (CO), another pollutant regulated under the Clean Air Act, for which the region met the standard almost 20 years ago.

Air quality in the Washington region has improved significantly since 1990 and will continue to do so for many more years, remaining well below current standards through 2040. Emissions of the pollutants responsible for ozone formation -- NOx and VOCs -- will by 2017 have fallen by around 80% since 1990, according to the latest forecasts.

The significant declines have been and will continue to be driven primarily by advances in vehicle technology, cleaner fuels, and new fuel efficiency standards. A slowdown in the growth of driving in recent years has also contributed somewhat to lower emissions, and will probably continue to do so in coming years.

Eventually, the full benefits of all currently anticipated advances in technology will be realized, at which point emissions will begin to rise again as more travel occurs in the region. According to the latest forecasts, this turnaround is likely to occur sometime between 2030 and 2040. As new scientific data about the health impacts of regulated air pollutants become available, the federal government may further reduce the pollutant concentrations it deems to be healthy, requiring the region to lower its budgets of allowable emissions. If the increases in emissions exceed adopted budgets, the region would run the risk of losing federal transportation funding if it is unable to mitigate the excess pollution by diverting funding into projects that would achieve sufficient further reductions.

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