• Air Toxics
• CMAQ
• Conformity
• It All Adds Up

• Laws and Regulations
• Maps
• Policy and Guidance
• Research
  • General Practice
  • Air Quality Planning and SIPs
  • Emissions Analysis
  • Land Use
  • Multi-Jurisdictional (Complex) Areas
  • Project Level Conformity
  • Small Urban and Rural Areas
  • Tools & Methodologies
  • Transportation Control Measures
• Resources
• Models & Methodologies
• Training
• Air Quality Newsletter

Multi-Pollutant Emissions Benefits of Transportation Strategies-FHWA

Appendix C: Overview of Emissions Factors and Assumptions Used in Sample Calculations

Methodological Procedures

Most strategies can be analyzed in multiple ways, and variations of these approaches are available. The methods described in this report are generally simple sketch planning approaches that involve two main processes: 1) estimating the travel, speed, or vehicle changes associated with the strategy; and 2) estimating emissions impacts. For those strategies where sketch planning was not appropriate, we used the COMMUTER Model and the National Inventory Model (NMIM) Model.

The most challenging part of each methodology is developing the values for changes in travel activity, speeds, or vehicle stock. Various approaches can be used for these inputs, such as using models (e.g., travel demand forecasting models, COMMUTER Model), original data collection (e.g., surveys), past experience/studies of similar projects, standardized factors drawn from the literature, or professional judgment.

Application of Emissions Factors

Unless otherwise noted in each of the strategy review sections, all of the on-road strategies analyzed and presented in this report use emissions factors generated from MOBILE6.2 to determine emissions impacts. Factors generated for PM-2.5, PM-10, CO, NOx, VOCs, SOx, and NH3 were used to estimate the emissions impacts of documented strategies. Standard defaults have been applied, covering years 2006, 2010, and 2020. Emissions were generated for start (trip-based factors assuming 100 percent cold start) and running emissions (per mile factors). The recognition of a difference between trip starts emissions and running emissions is significant, since emissions control equipment does not function as effectively from a "cold start" causing the release of more pollutants during the first few miles of a trip.

MOBILE6.2 allows the calculation of several different types of vehicle emissions. This is important for some strategies because they affect running and trip emissions differently, as noted in this report. In areas that are concerned with VOC emissions, MOBILE6.2 modeling can be tailored to account for only the emissions that are affected by a particular strategy. For example, a trip reduction strategy not only reduces running emissions, but also start and "hot soak" emissions. Strategies that shorten trip length but do not eliminate trips affect running exhaust and running loss emissions, but not start, hot soak or diurnal soak emissions. Finally, most modeling should employ the "NO REFUELING" command in MOBILE6.2, since refueling emissions are associated with gas stations and are not normally affected by the types of stategies outlined in this document. Guidance is available from EPA and FHWA for those wishing to refine their analyses.

This section discusses the emissions factors used to perform the sample calculations.

MOBILE6.2 Emissions Factors

The major input parameters used to generate running mobile emissions factors for CO, PM-10, PM-2.5, NOx, VOCs, SOx, and NH3 are shown in the table below.

Table C-1. Major Input Parameters for MOBILE6.2 Emissions Factor Modeling