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Recent Examinations of Mobile Source Air Toxics

A Methodology for Evaluating Mobile Source Air Toxic Emissions Among Transportation Project Alternatives


Using parameters that reflect realistic conditions versus rare or unlikely events results in emission factors in the bottom half of the predictive range of MOBILE6.2. For the most part, these emission factors are nevertheless sensitive to changes in calendar year, congested speed, temperature, and fuel RVP. MSAT emission factors decrease with increasing calendar year, reaching their minimum during 2035 and remaining flat through 2050.

The results obtained for diesel particulate matter are not characteristic of those obtained for the HC-based MSATs (i.e., acetaldehyde, acrolein, benzene, butadiene, and formaldehyde). As illustrated in each of the figures, emission factors for diesel particulate matter are insensitive to changes in vehicle speed, temperature, and not surprisingly, fuel (gasoline) RVP. Emission factors for diesel particulate matter will only change as a function of the VMT of diesel-fueled vehicles and the diesel-fuel sulfur content, which will be set for future years by EPA regulation.

For the HC-based MSATs, higher emission factors are associated with lower operating speeds. Because a higher frequency of lower speeds is linked with traffic on arterials, higher emission factors are generally obtained for arterial versus freeway travel. The differences in drive cycles for the arterial versus freeway roadway scenarios have little to no effect. Higher emission factors are obtained for higher and lower temperatures with 75 ºF being the inflection point. Temperature and fuel-RVP effects are inter-related. Changes in fuel RVP for moderate temperatures have a minor effect on emission factors. However, the misapplication of fuels for high or low temperatures will have an effect on predicted emission factors for the HC-based MSATs. Use of high volatility gasoline with high temperatures results in higher emission factors compared with the proper match of low volatility gasoline with high temperatures. The same is true for the use of low volatility gasoline with low temperatures compared with the appropriate match of high volatility gasoline with low temperatures.


This study provides some insight into what may be expected when conducting an in-depth project-level mobile source air toxics emissions analysis. First, the main analytical tool for predicting emissions from on-road motor vehicles is the EPA's MOBILE6.2 model. The MOBILE6.2 model is regional in scope and has limited applicability to a project corridor. However, the effects of a major transportation project extend beyond its corridor and an evaluation within the context of an affected transportation network can be accomplished.

When evaluating the future options for upgrading a transportation corridor, the major mitigating factor in reducing mobile source air toxic emissions is the implementation EPA's new motor vehicle emission control standards. Substantial decreases in MSAT emissions will be realized from a current base-year through an estimated time of completion for a planned upgrading project and its design year some 25 years in the future. Even accounting for anticipated increases in vehicle-miles of travel and varying degrees of efficiency of vehicle operation, total MSAT emissions were predicted to decline more than 56% from 2005 to 2030. While benzene emissions were predicted to decline more than 41%, emissions of diesel particulate matter were predicted to decline more than twice this rate (i.e., 88%). On a toxicity-weighted basis, the effective decrease in total MSAT emissions is 81% from current to design year levels.

The ability to discern remarkable differences in MSAT emissions among transportation alternatives is difficult given the uncertainties associated forecasting travel activity and air emissions 25 years or more into the future. In this hypothetical congestion-mitigation project, differences in MSAT emissions between the Build and No-Action Alternates ranged from 2 to 6%. While factors such as ambient temperature, implementation of an inspection maintenance program, use of reformulated gasoline, etc., can affect the magnitude of MSAT emissions specific to a locale; these factors would be common to all project alternatives under review.

The most important factors affecting emission differences among the available options are vehicle-miles of travel and levels of traffic congestion. When evaluating transportation network alternatives operating significantly under-capacity, the difference in vehicle-miles of travel is more important than the difference in congested vehicle speeds. The excess capacity would accommodate an increase in traffic volumes without adversely affecting travel speeds and related MOBILE6.2 emission factors. At the other extreme, where one transportation network alternative is operating significantly over its capacity, then the difference in congested vehicle speeds may be more influential than the difference in vehicle-miles of travel. MOBILE6.2 emission factors are very sensitive to vehicle speeds in the slow, congested speed range. Mitigating this congestion may have more of an effect on reducing emissions than the offset due to a potential increase in vehicle-miles of travel. For transportation network alternatives operating slightly under- or over-capacity, then differences in vehicle-miles of travel and differences in congested speeds are equally significant. The level of detail required in formulating vehicle activity data is greater for congestion-mitigation projects. Factors that may mitigate or adversely affect congestion need to be accounted for and it is preferable to represent congestion by an hour-by-hour variation in traffic speeds versus an average for the day.

Applicability to Real-World Analyses

The approach used in this analysis could be applied for project-level analysis of proposed projects in the National Environmental Policy Act (NEPA) process, or for other purposes. However, the analysis needs to be tailored to reflect local conditions.

The geographic area of analysis should reflect, at a minimum, all roadways where traffic volumes are affected by the proposed project. The affected transportation network can be defined as those links where the AADT is expected to change by more than ±5% as a result of a project. Also, to better reflect total emissions of the six priority MSATs, the analysis could include not only emissions associated with the transportation facilities in question, but also emissions from the local street network, non-road mobile sources, area and point (industrial) sources. Including these other emissions sources provides a more accurate representation of the relative impact of the proposed project.

This analysis is based on assumptions regarding traffic volumes and V-to-C ratios. An actual analysis would use volumes and capacity information specific to the project. Rather than using arbitrary growth rates, future volumes should be projected using a travel demand model or other technique normally used to forecast future travel in the area. Speeds from the travel demand model can also be used, but they should be post-processed using the TTI methodology, Bureau of Public Roads (BPR) formula, or other methodology. An enhancement would be to account for the effects of lower levels of weekend travel.

This analysis is based largely on national defaults in the MOBILE6.2 model. An actual analysis would use MOBILE inputs that are appropriate to the area. To a large extent, these inputs should be consistent with those used for other modeling purposes in the area (e.g., State Implementation Plan inventories, conformity analyses). However, given the limitations of the accuracy of the MOBILE6.2 model, use of annual average inputs is probably appropriate for most analyses. Also, rather than modeling each individual speed calculated for project links, it may be more expedient to generate a speed look-up table, in 5 mph increments, and select emissions rates by rounding to the closest modeled speed. Also note previous comments regarding use of hourly speeds versus daily average speeds. In many cases, daily average speeds would be appropriate.


The content of this paper solely represents the work of the authors and does not reflect the policy, guidance, or procedures adopted or recommended by the U.S. Department of Transportation and the Federal Highway Administration. This document is disseminated in the interest of information exchange and the U.S. Government assumes no liability for use of the information. This paper does not constitute a standard specification or regulation.


  1. BTS, 2003, Annual Vehicle Miles Traveled,
  2. EPA, 2004, "Technical Guidance on the Use of MOBILE6.2 for Emission Inventory Preparation", EPA420-R-04-13, Office of Transportation and Air Quality, August 2004.
  3. EPA, 2003, "User's Guide to MOBILE6.1 and MOBILE6.2, Mobile Source Emission Factor Model", EPA420-R-03-010, Office of Transportation and Air Quality, August 2003.
  4. NHI, 2003, "Estimating Regional Mobile Source Emissions", NHI Course Number 152071, U.S. Department of Transportation, Federal Highway Administration, October 2003.
  5. Tang, T., et. al., 2003, "MOBILE6.2 Air Toxic Trend and Sensitivity Analysis", U.S. Department of Transportation, Federal Highway Administration, Resource Center.
  6. TRB, 2000, Highway Capacity Manual 2000, Committee on Highway Capacity and Quality of Service.
Updated: 7/6/2011
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