Section 5: Air Quality Conformity Modeling

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Environmental Protection Agency (EPA) regulations require that non-attainment area transportation plans are in conformity with the National Ambient Air Quality Standards (NAAQS) (accessible from a link on the TxDOT Internet at Areas classified as non-attainment under the CAAA of 1990 must conduct modeling as specified in the federal air quality conformity regulations (accessible from a link on the TxDOT Internet at The modeling requirements specify that plans must be based on:

  • travel demand models to estimate how much travel will occur in the region based on the latest travel characteristics and growth assumptions
  • the latest emissions models to estimate regional emissions based on the output of the travel model
  • air quality dispersion models to evaluate localized impacts of carbon monoxide emissions.

For areas classified as serious, severe, and extreme ozone non-attainment and/or as serious CO non-attainment areas, there are additional specific modeling requirements:

  • Planners must validate the network-based model against ground counts for a base year that is not more than 10 years prior to the date of the conformity determination.
  • The model must employ a capacity sensitive assignment technique for peak-hour or peak-period traffic assignments.
  • Analyses must base free-flow speeds of network links on empirical observations.
  • The model must provide peak and off-peak travel demand and travel times.
  • Zone-to-zone travel times used to distribute trips between origin and destination pairs must be in reasonable agreement with the travel times resulting from the assignment of trips to the network links.

Subsequent to travel demand modeling, TxDOT uses a series of computer programs to perform the required air quality conformity analysis. These are discussed below.

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Air Quality Conformity Modeling Process and Programs

The air quality modeling process (see Figure 2-3) involves a series of computer programs in conjunction with output from the travel demand models. The process uses a loaded network file and 24-hour trip table, both output from the travel demand modeling process, in conjunction with additional data inputs to develop gridded emissions for urban areas classified as non-attainment. The following paragraphs summarize specific model components and data inputs.

Air Quality Modeling Process. To see a
PDF file of the process, click tda_2-3. (click in image to see full-size image) Anchor: #i1004226

Figure 2-3. Air Quality Modeling Process. To see a PDF file of the process, click tda_2-3.

PREPIN. This program prepares the travel model and VMT related data for use by subsequent programs in air quality modeling. PREPIN requires the following data sets:

  • Transportation system network – A build and a no-build network
  • Origin-destination vehicle trip table from trip distribution in travel demand model
  • Equilibrium traffic assignment from traffic assignment from travel demand model
  • Zone radii cards (developed for trip distribution in travel demand model)
  • Zone to area type table of equals
  • Zone to county table of equals
  • VMT seasonal scale factor – Used to scale the estimated weekday traffic from the travel model to weekday traffic in the month of July. These factors are developed from ATR data.
  • HPMS scale factor – Base year travel model VMT must be scaled to the base year HPMS VMT. This factor is developed using county HPMS VMT and is applied to all forecasts prepared using the travel model.
  • Directional split factor – A factor used to split the total link volume for a particular time period into the link volume in each direction.
  • Time-of-day factor – Used to disaggregate the 24-hour link volume from the traffic assignment into time periods.
  • Link speed estimation model – Link volume, link capacity, the link free flow speed, and the link facility type are used by the speed model to estimate link speed. There are two models used in Texas: the Houston-Galveston Area Council model used in the Houston area and the North Central Texas Council of Governments (NCTCOG) model used in Dallas-Fort Worth, El Paso, and Beaumont-Port Arthur.

MOBILE5A. (see link on the TxDOT Internet at The EPA’s highway vehicle emission factor model used by TxDOT is MOBILE5A Hybrid. This is a FORTRAN program that provides average in-use fleet emission factors for volatile organic compounds (VOCs), carbon monoxide (CO), and oxides of nitrogen (NOx) for each of eight categories of vehicles. (see Table 2-5). The program develops emission factors for calendar years between 1970 and 2020 for various conditions affecting in-use emission levels such as ambient temperatures, average traffic speeds, and gasoline volatility. TPP uses this program in evaluating control strategies for highway mobile sources and in the development of emission inventories and control strategies for State Implementation Plans (SIPs) (link to SIP manual section) and in the development of environmental impact statements.

The output from the model is in the form of emission factors expressed as grams of pollutant per vehicle mile traveled (g/mi.). Emission factors from MOBILE5 are combined with estimates of total vehicle miles traveled to develop highway vehicle emission inventories in terms of tons per day, per month, per season, and per year. The model also provides a number of estimates of non-exhaust (non-tailpipe) VOC emission sources from gasoline-powered vehicles. These include diurnal emissions, hot soak emissions, running loss emissions, resting loss emissions, and refueling emissions. The program provides non-exhaust emissions in either g/mi. units or other units as appropriate for more detailed modeling requirements.

Anchor: #i1010057Table 2-5. Eight Categories of EPA Vehicle Types



Gross Vehicle Weight Range


Light duty gasoline vehicle

< 6,000


Light duty gasoline truck

< 6,000


Light duty gasoline truck

6,001 - 8,500


Heavy duty gasoline vehicle

> 8,500


Light duty diesel vehicle

< 6,000


Light duty diesel truck

6,001 - 8,500


Heavy duty diesel vehicle

> 8,500




POLFAC5A. TPP uses this program with EPA’s MOBILE5A program to develop emission rates for eight vehicle types and 63 speeds (3 mph through 65 mph) for each of the three primary pollutants:

  • carbon monoxide (CO)
  • volatile organic compounds (VOC)
  • nitrous oxide (NOx).

The appropriate model inputs to represent factors such as the inspection and maintenance program, reformulated gasoline, and low emission vehicles for MOBILE are determined cooperatively between the Texas Natural Resource Conservation Commission (TNRCC) and the MPO. TPP develops vehicle registration data from the TxDOT vehicle registration database for counties. VMT mix is developed from vehicle classification data.

IMPSUM. This program reads VMT records output from PREPIN, emission rate records output by POLFC5A, temperature rise records, VMT totals by county and time period, the node coordinates, and the county name records. The outputs of this program are emissions by county and grid square, and the sums of VMT, VHT, and average speed by county.

SUMALLA. This program reads the emissions for each time period and sums the emissions from all time periods to produce 24-hour emission summaries.

VMTSUM. This program reads the link records from all time periods and sums the VMT by time period and county.

RATADJ and RATEADJV. TPP uses these two programs when multiple runs of MOBILE5 are required to obtain the appropriate emission rate. Analysts select RATADJ when emission rates for all eight vehicle types are to be adjusted. They choose RATEADJV when some, but not all, of the eight vehicle type emission rates need to be adjusted.

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