TPB News

As Ozone Seasons Gets Underway, Air Quality Analysis of 2015 CLRP Update Kicks Off

Jun 2, 2015

Ozone season is underway in the Washington region. That means that area environmental agencies together with the Metropolitan Washington Council of Governments are monitoring concentrations of the harmful pollutant and issuing alerts for days when concentrations are expected to exceed healthy levels. Summer is the peak season for ground-level ozone because of the abundance of direct sunlight as well as more stable weather patterns, which can lead to greater air stagnation.

More: Get the latest air quality forecast and sign up for alerts

As it happens, engineers and planners at the Transportation Planning Board get busy during ozone seasons, too -- forecasting future vehicle-related emissions of the two main ingredients in ozone formation, nitrogen oxides (NOx) and volatile organic compounds (VOCs). These two pollutants are emitted from the tailpipes of most cars, trucks, and buses on Washington area roadways. In its analysis, the TPB forecasts future vehicle-related emissions of two other pollutants, too -- fine particles and carbon monoxide.

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The TPB carries out its annual emissions forecasts before it adopts updates to the region's Constrained Long-Range Transportation Plan, or CLRP. Federal law requires the forecasts in order to demonstrate that the plan supports rather than hinders regional efforts to improve air quality.

The forecasts taking place this summer are part of the 2015 CLRP update, which kicked off late last year when the TPB called on area transportation agencies to submit new projects or changes to existing projects to include in the plan.

In March, the TPB approved six major additions or changes to include in this summer's analysis, including new express toll lanes on I-66 in Northern Virginia. The changes also include several dedicated bicycle lane projects in the District of Columbia that will require a reduction in the number of travel lanes for automobiles. The projects join hundreds of other improvements already in the CLRP that either add or remove highway or transit capacity and could therefore affect future travel patterns and air quality.

More: See all six major additions and changes proposed for the 2015 CLRP update

To forecast future emissions under the CLRP, the TPB's engineers and planners must first predict where, when, and how people will travel in the future. To do so, they look at the changes to the transportation system that are outlined in the CLRP as well as anticipated population and job growth and where that growth is likely to occur.

Then the engineers and planners must consider how vehicle and fuel technologies are expected to change over that same time and how that will affect emissions.

To assist with estimating emissions, the US Environmental Protection Agency (EPA) provides a regularly updated computer model that takes into account the latest developments in vehicle technology, anticipated changes in technology under new federal regulations, changes in the formulation of vehicle fuel, and refinements to the scientific understanding of how vehicular emissions are generated.

The latest major update of the model, known as MOVES2014, incorporates several important changes that have occurred since the last major update of the model in 2010. New federal fuel efficiency standards mean cars and trucks are expected to achieve even higher gas mileage in the future than previously thought, yielding lower emissions. New fuel formula standards requiring lower sulfur content will mean lower emissions, too. And new scientific data have helped the EPA refine its estimates of emissions from cars and trucks under various operating conditions.

More: See the approved scope of work for this year's air quality analysis using MOVES2014

Together, the changes reflected in MOVES2014 help develop more accurate and more precise estimates of future emissions in the Washington region. Compared to last year's forecasts, those made using MOVES2014 are expected to show lower future emissions than were previously estimated.

The full results of the air quality analysis for the 2015 CLRP update are due later this year. They will be made available for public comment in September before the TPB considers the entire plan update for final approval in October.

For more information about the 2015 CLRP update, please visit


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