Since 1970, significant progress has been made in reducing mobile source emissions, despite increases in population, employment, and vehicle miles traveled. Most of the emissions reductions have resulted from cleaner vehicles and cleaner fuels. Transportation agencies have contributed to emission reductions through the implementation of transportation system management (TSM) strategies such as traffic signalization projects, and high occupancy vehicle lanes, and through transportation demand management (TDM) strategies such as transit facilities, and pedestrian and bicycle programs. Nevertheless, mobile sources are a major contributor to volatile organic compound (VOC), oxides of nitrogen (NOx), carbon monoxide (CO), and particulate matter (PM) emissions and remain an area of concern to regulatory agencies and to the general public. More recently, mobile source air toxics and greenhouse gas emissions have also become concerns. Transportation planners address air quality issues in two contexts: the regional and project scales. Regional-scale analyses estimate how regional transportation plan (RTP) and transportation improvement program (TIP) implementation affect region-wide emissions. During the transportation conformity process, these regional-scale emissions are often compared to allowable levels, or "budgets." Project-scale analyses involve "hot spot" assessments focused on whether pollutant concentrations near roads exceed health-based standards, or whether project implementation increases or decreases emissions.
- National Ambient Air Quality Standards
- State Implementation Plans
- Emissions Inventories and Motor Vehicle Emissions Budgets
- Control Strategies