Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Office of Planning, Environment, & Realty (HEP)
PlanningEnvironmentReal Estate

HEP Events Guidance Publications Awards Contacts

Transportation Air Quality Selected Facts and Figures

Emissions Standards

Despite continued improvements, the air quality issues facing states and regions require that policymakers consider strategies to reduce emissions from all sources - point and area, on-road vehicles, and non-road engines. The strategies available to transportation and air quality officials range from regulatory to voluntary, and from technology- and fuel-based strategies to market-based measures aimed at changing driver behavior. Two of the more common policy responses are described below.


Transportation conformity is a process to ensure that federal funding and approval are given to those transportation activities that are consistent with air quality goals. The conformity regulation requires that all transportation plans and programs in nonattainment or maintenance areas conform to the state's air quality plan, known as the State Implementation Plan or SIP. It ensures that transportation activities do not worsen air quality or interfere with the purpose of the SIP, which is to attain the National Ambient Air Quality Standards (NAAQS). Meeting the NAAQS often requires emissions reductions from mobile sources. Several transportation emissions reduction strategies are available and, in some regions, required to help regions attain the standards.

Inspection and Maintenance Programs

An Inspection and Maintenance Program (I&M) can help to identify excessive vehicle emissions so that the owner can get the vehicle repaired. I&M programs fall into two categories.

Technology Improvements

Emissions reductions can be achieved by improving engine technology or using alternative fuels or reformulated gasoline. Among engine improvements, the catalytic converter, which extracts pollution from exhaust, has made the largest contribution to reduce vehicles emissions in recent years. A catalytic converter does not operate effectively, however, until it reaches its operating temperature after a car has been running for a few minutes. High sulfur content in fuel, like lead in the 1970s, has been shown to reduce the effectiveness of the catalytic converter. To reduce these emissions, EPA has promulgated a low sulfur fuel rule in conjunction with Tier 2. Researchers are exploring ways to reduce the time needed to heat the catalytic converter. At least one manufacturer has changed the placement of the catalytic converter so that it heats up and functions more quickly.

Idle Reduction - there is a concerted effort to establish and fund projects that reduce continuous idling of heavy-duty diesel truck engines at truck stops. These projects provide electric power for cab space cooling and heating, as well as other amenities, such as Internet access and television viewing. Idle-reduction measures can help meet multiple goals, including energy conservation, energy security and environmental stewardship. There are several technologies available to reduce idling including direct-fired heaters, Auxiliary Power Units (APUs), automatic engine idle management systems, and Truck Stop Electrification (TSE).

Diesel Retrofit - the growth in interstate commerce has spawned a corresponding growth in diesel truck traffic. As new measures and strategies are implemented to help meet the motor-vehicle emissions budgets in SIPs, expanded application of the diesel engine retrofit program could be an opportunity in many areas. Several retrofit emissions control technologies are available with varying levels of demonstrated effectiveness at reducing PM, VOC, CO, and air toxics. The diesel particulate filter has the potential to reduce particulates to near zero. Advanced diesel oxidation catalysts result in less dramatic reductions, but are less expensive and can run on diesel with higher sulfur levels. Various fuel additives can also help reduce emissions of some pollutants compared to regular diesel fuel.

Use of Reformulated Fuels 1993-2002

Use of Reformulated Fuels 1993-2002. Click image for source data.

Oxygenates such as Methyl Tertiary Butyl Ether (MTBE) or Ethyl Tertiary Butyl Ether (ETBE) are blended with gasoline (reformulated fuel) to increase the oxygen content for more complete combustion in engines, resulting in decreased tailpipe emissions.

EPA has implemented two "cleaner burning" fuel programs. One is the Winter Oxyfuel Program that requires oxygenated fuel during the cold months in cities, which have elevated levels of carbon monoxide. Ethanol is the primary oxygenate used in this program, and once an area is redesignated to CO attainment, the use of oxygenated fuels becomes optional.

The Year-Round Reformulated Gasoline Program requires reformulated gasoline (RFG) year-round in cities with the worst ground-level ozone (smog). RFG is oxygenated fuel that is specially blended to have fewer polluting compounds than conventional gasoline.

As a result of its use in the nation's fuel supply, MTBE has gotten into the groundwater and created a risk to drinking water and ground water resources. Due to these concerns, Congress is considering a limit or ban on the use of MTBE as a fuel additive.

Source: U.S. Department of Energy, Energy Information Administration. Alternatives to Traditional Transportation Fuels 2000. Table 10. September 2002.
Web site:, 28 June 2005.

Source: U.S. Environmental Protection Agency. Legislative Principals for Protecting Drinking Water Supplies, Preserving Clean Air Benefits, and Promoting Renewable Fuels. March 2000.

Gasoline Requirements

Map of the United States showing the numerous counties that are using one of 20 different fuels. Click image for details.

Gasoline requirements vary across the country. The map above depicts the 20 different fuel requirements currently used in the United States.

Source: ExxonMobil. U.S. Gasoline Requirements. K.W. Gardner, January 2004. Used with permission.

Alternative Fuels

Vehicle/engine design is a critical factor affecting emissions from alternative fueled vehicles. A variety of alternative fuels are available and can be used to combat different air pollution problems:

Liquefied Petroleum Gas (LPG) - A fossil-fuel derivative composed of 95 percent propane and 5 percent butanes. It produces lower CO emissions, but NOx emissions may be higher.

Natural Gas - A fuel that can be in compressed (CNG) or liquefied (LNG). The CNG form, more common in the transportation sector, is stored in high-pressure cylinders. CNG generates lower CO and VOC emissions than conventional gasoline, and lower NOx and PM than diesel fuels.

Ethanol - Grain alcohol made from corn, sugarcane, or woody biomass. Ethanol blends may reduce CO emissions, but their effect on ozone is negligible.

Electricity - Electric vehicles may be powered by batteries either charged at home or at charging stations with electricity from power plants. They have no tailpipe emissions; overall emissions depend on power plant energy sources.

Hydrogen - There are two types of engines that burn hydrogen. One is an internal combustion engine, the other is a fuel cell. Hydrogen is clean-burning fuel that can be produced from coal, natural gas, petroleum, solar, or wind energy. A vehicle operating on a fuel cell, which generates electricity by harnessing the reaction of hydrogen and oxygen to make water, produces no CO or VOC emissions and extremely low NOx emissions.

Alternative Fueled Vehicles in Use 1995-2002

Alternative Fueled Vehicles in Use 1995-2002. Click image for source data.

Use of alternative fuels for motor vehicles has increased in recent years. More than 471,000 alternative fuel vehicles (AFVs) were on the road in 2002, a 90 percent increase since 1995. These increases are due to a number of policies including the Energy Policy Act of 1992, Presidential Executive Order 12844, which requires minimum AFVs purchases for vehicle fleets, and the availability of federal funding under the Congestion Mitigation and Air Quality Improvement (CMAQ) program. Energy policy mandates requiring state and fuel provider fleets to acquire AFVs also took effect starting in model year 1997.

Share of Alternative Fuel Vehicle by Fuel Type 2002

Share of Alternative Fuel by Fuel Type 2002. CNG 26%; Ethanol 26%; LPG 39%; other 9% (LNG 1%, Electricity 7%, Methanol 1%)

AFVs designed to operate on Liquefied Petroleum Gas (LPG) were the most popular in 2002, followed by CNG and ethanol.

Source: U.S. Department of Energy, Energy Information Administration. Alternatives to Traditional Transportation Fuels. Table 1. September 2002.
Web site: 28 June 2005.

Transportation Control Measures

States and localities can help reduce motor vehicle emissions by implementing measures to manage travel demand or improve traffic flow. Transpor-tation Control Measures (TCM) is the term used to refer to these efforts when they are included within a State Implementation Plan. Examples of these measures include:

Alternatives to Single Occupant Vehicle Travel
Measures that focus on providing alternatives to single-occupant vehicle travel, such as carpooling, transit, and bicycling:

Traffic Flow Measures
Measures that focus on improving the smoothness of traffic flow to reduce stop-and-go traffic conditions:

Market-Based Measures
Measures that rely on pricing as an incentive to reduce travel congestion:

Employer-Based Measures
Measures that employers implement:

Non-Traditional Measures

EPA's Voluntary Programs

EPA developed a number of voluntary programs aimed at reducing emissions from diesel vehicles on the road today. Some of these include Clean School Bus USA, SmartWay Transport, and the Diesel Retrofit and Replacement Program.


Numerous sources are available to fund measures that reduce transportation-related emissions,including traditional funding sources, state and local sources, user fees, and private sector resources. One major soure of funds, the Congestion Mitigation and Air Quality Improvement (CMAQ) program, was authorized under the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991, and reauthorized under the Transportation Equity Act for the 21st Century (TEA-21), and the Safe, Accountable, Flexible, and Efficient Transportation Act: - A Legacy for Users (SAFETEA-LU).

CMAQ Program

The CMAQ program allocates funds to states to implement transportation control measures and other strategies to help areas meet the NAAQS for ozone, CO and PM. The actual amount of funding is based on the severity of the air quality problem and the population of the area. State and local governments select the projects to fund and coordinate them through Metropolitan Planning Organizations. The projects vary by region, but typically include the following measures:

Other activities, such as idle reduction, diesel retrofit and education and outreach programs, may also be eligible for CMAQ funds if they contribute to reductions in on-road mobile source emissions.

Updated: 12/16/2015
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000