Mini Master Highlights

4: Traffic Data for Environmental Analysis

Environmental analysis is an integral part of the transportation project development process. Two topics relying heavily on traffic data are outlined below.

Topic # 4-1: Traffic Data for Transportation Air Quality Analysis

Transportation air quality analysis involves both regional and project level emissions assessment to meet federal regulatory requirements. One type of analysis required by the Clean Air Act is called regional conformity, which examines emissions from all vehicles in an air quality non-attainment or maintenance area. The computation of an "emissions inventory" involves the use of VMT, speeds, and fleet information developed from travel demand modeling output. This information, combined with other regional information on meteorology, fuels, age of fleet, etc., is used in an emissions model (either EPA's MOVES model or California Air Resources Board's EMFAC model, depending on the state) to calculate an emissions inventory and meet regional conformity requirements.    

Certain projects may also have to complete modeling to satisfy project level conformity requirements as specified in the Clean Air Act. These projects may require assessment of near-road air quality impacts for carbon monoxide and/or particulate matter. Similar to regional conformity, information on VMT, fleet mix, and speeds are provided to an emissions model, typically derived from a travel demand model. Emissions are calculated and used in a "dispersion" model to estimate pollutant concentrations around the project.

Other project level air quality activities may include calculating project-specific emissions inventories - including mobile source air toxics. These analyses similarly rely on traffic data from travel demand models processed through an emissions model.

The FHWA also deploys a database tool called Database for Air Quality and Noise Analysis (DANA). DANA can generate formatted inputs for the EPA MOVES model using traffic data from the National Performance Management Research Data Set (NPMRDS), Highway Performance Monitoring System (HPMS), and Travel Monitoring Analysis System (TMAS). Together, these sources provide speed, travel time, roadway characteristics, AADT, and vehicle classification information for estimating volumes, fleet mix, and emissions by vehicle type. The following summarizes the relevant traffic data.

Speed

The harmonic average speed for all reporting vehicles on the segment.

Travel Time

Travel time recorded in minutes or seconds. It is the ratio between the segment length and the harmonic average speed for all reporting vehicles on the segment.

HPMS_TYPE

Indicates the vehicle type percentages for HPMS vehicle types.

AADT

Annual Average Daily Traffic. If multiple HPMS segments with different attribute values are assigned to a single TMC path, the length-weighted average is assigned.

AADT (Single)

Annual Average Daily Traffic for single-unit trucks and buses. If multiple HPMS segments with different attribute values are assigned to a single TMC path, the length-weighted average is assigned.

AADT (Combination)

Annual Average Daily Traffic for Combination Trucks. If multiple HPMS segments with different attribute values are assigned to a single TMC path, the length-weighted average is assigned.

VMT by Roadway Function Class

Total Vehicle Miles Traveled (VMT) aggregated by rural and urban functional classes, including interstate, freeway, principal arterial, minor arterial, major collector, minor collector, and local roads. Used to calculate total VMT by road type for MOVES input files.

Topic # 4-2: Traffic Noise Assessment

Noise generated by vehicles operating on highways is assessed through FHWA's Traffic Noise Model (TNM). The model generates the hourly equivalent A-weighted sound level (LAeq(h)), defined as the equivalent steady-state sound level over a one-hour period. TNM relies on project-specific traffic data to provide accurate results.  Below is the traffic data needed to generate the LAeq(h). These traffic data components normally cover three-time horizons. 

Existing Data

• Existing condition traffic volumes by class (automobile, medium truck, heavy truck, bus, motorcycle).
• Existing condition vehicle speeds (by vehicle class or universal)

Opening Year Data

• Opening year refers to the year the highway will open to travel.
• Opening year traffic volumes by class (automobile, medium truck, heavy truck, bus, motorcycle
• Opening year vehicle speeds (by vehicle class or universal)

Design year data

• Design year refers to the year for which the highway is designed.
• Design year traffic volumes by class (automobile, medium truck, heavy truck, bus, motorcycle
• Design year vehicle speeds (by vehicle class or universal).