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. 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. 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. 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. 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. In addition to being uncertain about the amount of emissions related to road grade, EPA officials explained that obtaining the basic data from instrumented cars and chase cars to make such adjustments would be expensive at this time and that because of the cost and length of time required for these studies, the impact of road grade will probably not be addressed until MOBILE7. According to a 1993 CARB study, the FTP may underestimate hydrocarbon, carbon monoxide, and nitrogen oxide emissions by 27, 68, and 17 percent, respectively. However, as discussed below, the agency will not be able to quantify the uncertainty associated with its MOBILE model estimates, primarily because of the complexity and timing of factors affecting vehicle emissions and the high cost of vehicle studies. Comments From the Environmental Protection Agency

Objectives, Scope, and Methodology

The Chairman, Subcommittee on Oversight and Investigations, House Committee on Commerce, asked us to (1) describe the major limitations in the current version of EPA’s MOBILE model and (2) describe EPA’s process for improving both current and future versions of the MOBILE model.
Why GAO Did This Study

Pursuant to a congressional request, GAO reviewed the Environmental Protection Agency's (EPA) MOBILE series of complex computer models to estimate motor vehicle emissions, focusing on the model's major limitations and EPA's process for improving the current and future versions of the model.

What GAO Found

GAO noted that: (1) EPA and a group of stakeholders have identified 14 major limitations in the current MOBILE model; (2) some vehicle emissions-producing activities are not accounted for in the current model, and other emissions-producing activities may not be adequately represented on the basis of the most recent information; (3) according to EPA, much of this information has become available since MOBILE5a was released; (4) these limitations cause the model to underestimate vehicle emissions in some cases and overestimate them in others; (5) other studies indicate that some activities are inadequately represented in the model; (6) another study indicates that carbon monoxide and hydrocarbon emissions from higher mileage vehicles may be significantly less than the model's estimates; (7) EPA plans to address most of these limitations in its next revision to the MOBILE model, however, according to agency officials, three of the limitations will probably not be addressed until later because of a combination of factors; (8) according to agency officials, these include the negligible impact on emissions inventory predictions, a relatively low priority ascribed by EPA and stakeholders, the cost and length of time required for these studies relative to the schedule for release of MOBILE6, and the emergence of new technologies that will make the improvements more feasible or cost effective in a few years; (9) EPA officials pointed out that they have updated the estimating capabilities of the MOBILE model 10 times since it was first introduced in 1978; (10) irrespective of these limitations, there are specific actions, most of which were recommended by the Science Advisory Board in its 1989 resolution, that, when followed, can enhance a model's estimating capabilities; (11) among other things, these actions involve documenting the implicit and explicit assumptions that are the basis of the formulas contained in the model, obtaining external stakeholders' input during the model's development, and having the model peer reviewed before it is used; (12) EPA officials acknowledged that, primarily because of resource limitations, until recently such actions have been delayed or forgone; (13) however, EPA is developing the next model, MOBILE6, with significantly increased openness and input from other stakeholders; and (14) EPA also plans to carry out the actions recommended by the Science Advisory Board, such as peer review, as part of its program for developing MOBILE6, due to be issued in late 1998.