Publication Detail

Speed Correction Curves: Scoping Study


Research Report

Suggested Citation:
Niemeier, Debbie A., Jessica M. Utts, K. Nanzetta, Laura Ring (1998) Speed Correction Curves: Scoping Study. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-98-12

The Clean Air Act Amendments (CAAA) established a process requiring non-attainment areas to reduce emissions according to the National Ambient Air Quality Standards (NAAQS). Emissions are divided into three categories: mobile, stationary, and area sources. Emissions limits, or budgets, for each category are established in the State Implementation Plan (SIP). The Clean Air Act also requires that transportation plans, programs, and projects conform to the emissions budgets established in the SIP. That is, transportation activities must not cause or contribute to new violations, worsen existing violations, or delay attainment of the air quality standards.

Recently both the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) have proposed major modifications to their emissions modeling methods, MOBILE6 and EMFACX respectively. In particular, major revisions in the speed correction factors (SCFs) have been proposed. The purpose of this report is to examine the theoretical and methodological basis for the new modeling approaches, with respect to the development of new speed correction factors. As part of this effort, both the data used to develop the SCFs and the possible implications from a transportation perspective of implementing the new speed correction factors are evaluated. On the basis of this evaluation, recommendations will be made on the preferred method and on additional research and data collection that should be undertaken to enhance the new methodology.


Typically, state air quality agencies are charged with developing the SIP and the associated emissions budgets. Data for these emission budgets can come from a variety of sources including state departments of transportation (DOTs), metropolitan planning agencies (MPOs), and federal sources such as the Highway Performance Monitoring System (HPMS). In contrast to the SIP preparation, DOTs, MPOs, and local governments are usually required to perform the requisite conformity analyses. MPOs and local governments usually demonstrate conformity for transportation plans and programs, while DOTs often undertake project-specific conformity analysis.

Most of the modeling protocol, such as the California Air Resources Board's motor vehicle emissions inventory (MVEI), was originally developed as a means for preparing region-level air quality attainment plans and budgets. To comply with the conformity requirement, MPOs, DOTs, and local governments frequently rely on this modeling protocol to estimate emissions, even though the models were not developed or intended for this purpose.

The CAAA charged that closer examination of "real-world driving" be undertaken. In addition, the CAAA required the EPA to develop a revised Federal Test Procedure (FTP) cycle for certification purposes; emission inventory issues were not considered during cycle development. The revised cycle includes worst case conditions and does not represent the variety of in-use fleet driving patterns. As part of the FTP Review Project, EPA collected both chase car data and instrumented vehicle data in Baltimore, MD, and Spokane, WA. These data were supplemented by instrumented vehicle data collected in Atlanta, GA and chase car data from Los Angeles, CA.

Both EPA and CARB have recently proposed changes to the emissions modeling protocol, most notably that of the speed correction factors contained in MOBILE and EMFAC. Both models have traditionally used data from FTP certification cycle testing and correction factors for various conditions (e.g., average speed, temperature, and fuels) are applied to emissions measured at the FTP "standard" conditions. The speed correction factors are based on test results for vehicles tested on both the FTP driving cycle (and the DC cycle for CARB) in addition to several other cycles, each with a different average speed.

Report Organization

This report is organized into four chapters. In Chapters 2 and 3, we present an overview of EPA's and CARB's proposed SCF methodologies. In turn, we discuss the technical aspects of developing the respective driving cycles, the data collection and statistical issues involved in cycle development and SCF computations, and finally, the issues associated with potential application in the conformity process. The report concludes with a discussion of data collection research needs.