Quoted from: https://www.epa.gov/moves/description-and-history-mobile-highway-vehicle-emission-factor-model
MOBILE calculates emissions of hydrocarbons (HC), oxides of nitrogen (NOx) and carbon monoxide (CO) from passenger cars, motorcycles, light- and heavy-duty trucks. MOBILE is based on emissions testing of tens of thousands of vehicles. The model accounts for the emission impacts of factors such as changes in vehicle emission standards, changes in vehicle populations and activity, and variation in local conditions such as temperature, humidity and fuel quality.
MOBILE results were used to calculate current and future inventories of these emissions at the national and local level. These inventories were used to make decisions about air pollution policy and programs at the local, state and national level. Inventories based on MOBILE were also used to meet the federal Clean Air Act's state implementation plan (SIP) and transportation conformity requirements, and were sometimes used to meet requirements of the National Environmental Protection Act (NEPA).
The MOBILE model was first developed as MOBILE1 in the late 1970s, and has been updated periodically to reflect improved data, changes in vehicle, engine, and emission control system technologies, changes in applicable regulations and emission standards and test procedures, and improved understanding of in-use emission levels and the factors that influence them.
MOBILE estimates emissions of both exhaust and evaporative emissions. The output from the model is in the form of emission factors expressed as grams of pollutant per vehicle per hour (g/hr), or per vehicle mile traveled (g/mi). Thus, emission factors from MOBILE can be combined with estimates of total vehicle miles traveled (VMT) to develop highway vehicle emission inventories (in terms of tons per day, per month, per season, per year). The change in emission factors for a given vehicle category over time are a reflection of the impacts of fleet turnover. (Over time, older vehicles built to less stringent emission standards are replaced in the fleet by newer vehicles built in compliance with more stringent standards.)
Each generation of the MOBILE model became more sophisticated in its approach to modeling average in-use emissions and has provided the user with additional options for tailoring emission factor estimates to specific times and geographic locations. A brief history of the MOBILE model versions follows.