The allocation of emissions reductions and control strategies results in an emission reduction target for all sources. For on-road mobile sources, this target can be translated into an area's motor vehicle emissions budget (MVEB), which identifies the allowable on-road emissions levels to attain the air quality standards. These budgets are, in effect, a cap on emissions and represent the "holding capacity" of the area. Although these budgets are based on the emissions inventory projections, they may not be identical. For example, an area may wish to allocate a portion of its safety margin to its MVEB. The motor vehicle emissions budget that is explicitly identified in the SIP will be used in the transportation conformity process to cap the emissions allowed by motor vehicles on the regional transportation network as planned.
Transportation interests must be represented to appropriately weigh the trade-offs involved in the allocation of emission reductions between stationary, area and mobile sources. Transportation agencies have a strong interest in the development of budgets because motor vehicle emissions budgets can be a controlling factor in the development of transportation plans, programs, and projects.