Project Name

A Methodology to Estimate Vehicle Miles Traveled (VMT) Fractions as an Input to the MOBILE Emission Model

Research Team

Brian Smith

Yi Qi

Hyungjun Park


Virginia Center for Transportation Innovation and Research

Other Sponsors

US Environmental Protection Agency

Project Dates

August 2004 - December 2005

Project Description

Air quality has been a growing issue in the transportation sector since the enactment of legislation such as the Clean Air Act Amendments (CAAA) of 1990 and the Transportation Equity Act for the 21st Century (TEA21) in 1998. According to these acts, states and local governments are required to attain and maintain National Ambient Air Quality Standards. The MOBILE model is the mobile emission factor model used in estimating air pollutants generated by mobile sources. In order to obtain accurate emission estimates, it is essential to utilize sound input data that accurately reflects local conditions. Among many input factors, Vehicle Miles Traveled (VMT) fractions – the percentage of VMT for each vehicle type by roadway functional class – plays a critical role.

In this study, a new methodology for estimating locally specific VMT fractions as an input to the MOBILE model was developed. Based on this methodology, VMT fractions were then computed for the six nonattainment areas in Virginia: Frederick County, Fredericksburg, Hampton Roads, Northern Virginia (NOVA), Richmond and Roanoke, and compared with fractions estimated by existing methodologies.  The comparison revealed significant differences.  These differences, coupled with the fact that the proposed methodology uses significantly more local data and requires fewer assumptions that existing methods, illustrate the need for VDOT to reconsider its approach to applying the MOBILE model.

Based on the results of this research effort, the following recommendation is presented to VDOT:

  • VDOT’s Environmental Division should utilize the proposed VMT fraction estimation methodology to generate input to the MOBILE model for mobile source emission estimates.


In order to meet the objectives of this research, the following tasks were completed.  Note that the activities of Tasks 2, 3, and 4 resulted in the new methodology proposed for estimating VMT fractions to input into the MOBILE model.

  • Review of the state-of-the-practice – A review of VDOT’s current practice in estimating VMT fractions was first conducted.  Then, the research team reviewed the two most widely used methodologies for estimating VMT fractions.  One is the method described in the report Use of Locality-Specific Transportation Data for the Development of MOBILE Source Emissions Inventories prepared by Cambridge Systematics in 1996. This method is referred to as the CS method in this report. The other method was presented in NCHRP Report 394 Improving Transportation Data for Mobile Source Emission Estimates in 1997. This is referred to as the NCHRP394 method in this report.
  • Develop a Locally Specific Vehicle-Mapping Table – VMT is generally reported in the FHWA vehicle types that is based on vehicle axle arrangement and the number of units (i.e. a tractor-trailer would be a 2 unit vehicle).  On the other hand, the vehicle classifications required by MOBILE are primarily based on the vehicle weight and fuel type. The sets of the FHWA vehicle types and the MOBILE vehicle types are provided in Table 1 and Table 2 in the final report, respectively. Given the differences evident in the tables, a vehicle-mapping table that can convert the VMT fractions from FHWA types to MOBILE vehicle types is necessary. Although the EPA has recommended a general vehicle-mapping approach using national default values, a regional vehicle-mapping table is desirable since vehicle fleet composition varies among different locations. In this task, a methodology was developed to derive locally specific vehicle mapping tables based on the vehicle registration data from the Department of Motor Vehicles (DMV) and the Vehicle Inventory and Use Survey (VIUS) data.
  • Derive VMT Fractions of FHWA Vehicle Types by Roadway Functional Classes Using Traffic Monitoring System (TMS) Data – Based on VDOT’s Traffic Monitoring System (TMS) data collected by the Traffic Engineering Division, the VMT for 13 FHWA vehicle types were obtained by roadway functional classes for each nonattainment area.
  • Estimate VMT fractions for MOBILE Vehicle Types by Roadway Functional Classes – First, the VMT fractions for FHWA vehicle types derived in Task 3 were converted to the VMT fractions for MOBILE5 vehicle types based on the vehicle-mapping table developed in Task 2.  At the conclusion of this task, the VMT fraction estimated using the new methodology was compared with the VMT fractions currently used by VDOT for estimating the mobile source emissions in the six nonattainment areas (VDOT’s 2002 Traffic Trend Report).  In addition, using the guidance provided in User’s Guide to MOBILE6.1 and MOBILE6.2 – Mobile Source Emission Factor Model, MOBILE6 VMT fractions were calculated and presented.