The current MOBILE model, MOBILE5a, also known as the EPA mobile source emissions factor model, is a computer program that estimates the emissions of carbon monoxide, hydrocarbons, and nitrogen oxides for eight different types of gasoline-fueled and diesel highway motor vehicles. The model consists of an integrated collection of mathematical equations and assumptions about the emissions from vehicles manufactured from 1960 to 2020; generally, the cars produced in the 25 most recent model years are assumed to be in operation in any given calendar year. The first MOBILE model was made available for use in 1978; since that time, major updates and improvements to the model have been made as more has become known about the complexity of the factors affecting vehicle emissions, as measurement devices have improved, and as more data have been collected. According to agency officials, these improvements have resulted in the refinement of emissions estimates for evaporative emissions (such as occur when the fuel tank and fuel system heat up on a hot summer day); for the uncorrected in-use deterioration (wear and tear) that results from poor vehicle maintenance or tampering; and for other factors. In its simplest form, EPA’s MOBILE model allows the model user to produce a number—an estimated quantity of emissions for the three pollutants of concern—by multiplying the estimated emissions per mile for an average urban trip times the estimated number of trip miles traveled in an area. Over the years, however, researchers have learned that vehicle emissions are highly complex. For example, EPA and others have indications today that as much as half of all hydrocarbon emissions from motor vehicles are evaporative emissions, under certain conditions. To compensate for the complexities of these and other emissions-producing activities, EPA has periodically adjusted its basic formula—through the use of revised “correction factors”—to approximate vehicle exhaust emissions in a range of situations. In essence, the correction factor is a multiplier added to the basic formula (miles traveled times emissions rate per mile) to adjust the model’s output to more closely reflect actual emissions. Except for California, EPA supplies the baseline emissions rates and correction factors for other model users—primarily state and local agencies—that typically supply their own estimates of the number of vehicle miles traveled, according to agency officials, as well as many other local area parameters, such as the average ambient temperature, vehicle classifications, and types of fuels sold. The MOBILE model exists because precise information about the emissions behavior of the approximately 200 million vehicles in use in the United States is not known, yet the need exists to estimate the impact of motor vehicles on air quality. For the states, the MOBILE model is a tool for constructing emissions inventories, creating control strategies, producing state implementation plans (SIP), and—subsequently— demonstrating control strategy effectiveness to EPA and others. For example, the states are allowed to vary a number of control strategy features, including the types of fuels used, the type of inspection and maintenance (I&M) testing network, the frequency of I&M testing, the ages and types of vehicles to be inspected, the stringency of the tailpipe test, the number and percent of inspected vehicles that may receive a waiver,and a host of other factors. The states may choose among a number of control options as long as the state’s control strategy achieves at least as many reductions as required by the Clean Air Act. For EPA, the MOBILE model is a tool for evaluating the adequacy of a state’s emissions inventory estimate, motor vehicle control strategies, and implementation plans. In essence, the model’s estimates provide EPA regulators with critical information that is used to evaluate the adequacy of a state’s program and the relative benefits of various policies to control motor vehicle emissions. Additionally, the model’s estimates can affect state policy decisions on issues such as the content and volatility of fuels, and some decisions on highway improvement projects. For example, the 1991 Intermodal Surface Transportation Efficiency Act (ISTEA) required, among other things, that state transportation improvement programs in certain nonattainment areas conform with the applicable state implementation plan developed under the Clean Air Act. Although the model’s original purpose was to support the development of mobile source emissions inventories, over the years its role and influence have been expanded considerably. Today, its estimates have a substantial influence not only on state and local programs but also on the automobile and oil industries, environmental and trade organizations, the public, and others. According to estimates derived partly from the MOBILE model, motor vehicles produce about 90 percent of the carbon monoxide, 50 percent of the hydrocarbons, and 30 percent of the nitrogen oxides emitted annually in major urban areas. EPA officials are examining 14 areas in the current MOBILE model in which major limitations exist. According to agency officials, it is their plan for each new version of the MOBILE model to reflect the most recent testing, data collection, and research that are available. They pointed out that EPA has updated the estimating capabilities of its MOBILE source emissions model 10 times since the model was first introduced in 1978. Table 1 briefly summarizes the areas in which major limitations exist, as well as EPA’s plans to address these limitations in its next revision to the model, MOBILE6, due to be issued in late 1998. (Additional information on these 14 areas is provided in app. I.) While acknowledging that some vehicle emissions-producing activities are not accounted for in the current model and that other emissions-producing activities are not adequately represented in the current model on the basis of the most recent information, EPA officials said that it is important to note that EPA has conducted and/or partially funded some of the studies that have led to the new data that now question the old estimates and assumptions. Additionally, they said that EPA has work under way to address most of these limitations. For example, since its formation in September 1995, an EPA-sponsored Federal Advisory Committee Act (FACA) subcommittee workgroup has identified 47 high-priority items for improvement in the current model, and EPA and workgroup representatives are examining these limitations. EPA is also in the process of developing new procedures for improving models in general, which are discussed below. Several model experts told us that it is the nature of models such as MOBILE5a to have limitations and to be in a continuous improvement mode. Agency officials agreed with this assessment, noting that the current model is better than any previous versions and reflects consistent growth in the quality and quantity of information available on very complex issues. Additionally, they pointed out that—through the FACA workgroup process—the revisions to the next MOBILE model, MOBILE6, have been undertaken with significantly increased openness and input from other government agencies, academia, the automobile and oil industries, environmental groups, and others. Our contacts with representatives of these groups confirmed this increased level of external stakeholders’ involvement in preparation for MOBILE6. Several commended EPA’s efforts in recent years to reach out to persons outside of the agency, and some noted that the outreach effort had given them a much greater appreciation of the model. While acknowledging that, historically, there have been few firm criteria on the processes that should be followed when creating or revising a model such as the MOBILE model, the executive director of EPA’s Science Advisory Board (SAB) told us that the agency has a project under way to develop agencywide procedures for improving models. According to the project director, the Office of Research and Development (ORD) is planning a workshop in December 1997 to discuss the status of modeling across several media and other modeling issues, including the need for better agencywide modeling procedures. The project director also said that the Science Advisory Board’s January 1989 resolution on models was one of the best documents available on the processes for creating and improving models in general. The SAB executive director and the ORD project director told us that in their opinion, there are specific actions—most of which were recommended by the SAB in its 1989 resolution—that, when followed, can enhance a model’s predictive capabilities. Among other things, these actions include the following: Obtaining external stakeholders’ input to ensure that the model’s assumptions and formulas receive critical review by those not involved in the model’s development. Documenting the implicit and explicit assumptions so that others can evaluate the basis of the formulas embedded in the model. Performing sensitivity analyses over key parameters to identify the most sensitive parameters and to establish the areas most in need of further research. Verifying the adequacy of the model’s mathematical code. Testing the model’s predictions with laboratory and field data to confirm that the model generates results consistent with its underlying theory. Conducting peer review to enhance the quality, credibility, and acceptability of the model’s applications. The SAB executive director pointed out that because of continuing concerns with the quasi-regulatory use of agency models, EPA issued agencywide guidance in 1994 specifically calling for the peer review of such models. This directive was a follow-on to EPA’s January 1993 agencywide policy requiring peer review of the scientific and technical work products used to support agency decisions. In September 1996 and March 1997, we reported and testified on the uneven implementation of EPA’s peer review policy, including that the MOBILE model had not been peer reviewed. EPA agreed with our recommendations for educating staff and managers about the merits of and procedures for conducting peer review and for ensuring that all relevant products are considered for peer review. The agency has set in motion a three-pronged approach to improve the implementation of peer review agencywide, including peer review of the next MOBILE model. According to the ORD project director, it is too early in EPA’s study to predict whether the agency may recommend that EPA require its offices to follow the other actions listed above when creating or revising models.