Wednesday, April 22, 2015

April Question of the Month

Question of the Month: 

What are the weight limits for heavy-duty vehicles on interstate highways? What weight limit exemptions exist for vehicles equipped with idle reduction technology?

Answer: Under federal law, no vehicle weighing more than 20,000 pounds (lbs) on one axle, 34,000 lbs on a tandem axle, or 80,000 lbs overall may access federal interstate highways (e.g., Interstate 70, which runs across the country from Maryland to Utah), regardless of where they get on the highway.[1] States must enforce these requirements, or they may not be eligible for federal highway funding. However, the U.S. Department of Transportation (DOT) allows states to offer weight-limit exemptions for heavy-duty vehicles (HDVs) with on-board idle reduction technology.

Please note that states may set their own weight restrictions for roads that start and end within their boundaries, but we will focus on interstate highway requirements here.

Idle Reduction Technologies
Federal regulations allow states to adopt weight exemptions for auxiliary power units (APUs) or other qualified technologies that reduce fuel consumption and tailpipe emissions from engine idling. APUs are portable, vehicle-mounted systems that provide power for climate control and electrical devices without idling. For long-haul trucks, these systems typically have a small internal combustion engine (usually diesel) equipped with a generator to provide electricity and heat. Other on-board idle reduction technologies include automatic start-stop controls, energy recovery systems, fuel-operated heaters, coolant heaters, and battery-electric and thermal-storage air conditioners.

State Weight Exemptions
States may permit HDVs equipped with idle reduction technology to exceed the specified weight limit by up to 550 lbs to compensate for the additional weight of the equipment. The allowance was previously 400 lbs, but the federal Moving Ahead for Progress in the 21st Century (MAP-21) legislation, enacted in 2012, increased it to 550 lbs. States must enact a law or institute an enforcement policy with their own exemptions to reflect this increased weight allowance. A map of APU weight exemptions by state is available on the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) State Recognition of the Auxiliary Power Weight Exemption to Gross Vehicle Weight website ( Vehicle weight limit exemptions for APUs are also displayed in the table below. As the map and table show, many states have not updated their laws and enforcement policies to reflect the increase in the federal allowance to 550 lbs, which means the exemption is still limited to 400 lbs. There are also six states where the exemption is not permitted at all.

APU Weight Exemption
State Implementation
550 lbs
State Legislation
400 lbs
State Enforcement Policy
State Legislation
State Legislation
* West Virginia Code 17C-13A-4 refers to the U.S. Code directly for the exact weight.

States must require HDV drivers to demonstrate eligibility for vehicle weight limit exemptions. For example, drivers may need to have paperwork on hand that verifies the weight of the idle reduction equipment and be able to demonstrate that it is functional. Requirements are different from state to state.

More information on these state weight limit exemptions is also available on the Alternative Fuels Data Center (AFDC) Laws and Incentives database ( The Advanced Search options ( allow you to identify specific exemptions by location, technology/fuel type (idle reduction), incentive/regulation type (exemption), and user-type (vehicle owner or driver). Each description of a state idle reduction weight exemption includes a reference to the applicable legislation or policy.

Refer to the following for more information on idle reduction technologies and state vehicle weight limit exemptions for this equipment:
·         EERE National Idling Reduction Network News (
·         AFDC’s Onboard Idle Reduction Equipment for Heavy-Duty Trucks page (
·         Argonne National Laboratory’s Idle Reduction Tools and Outreach Materials (

Clean Cities Technical Response Service Team

March Question of the Month

Question of the Month:  

Question of the Month: What are the key terms and considerations I should remember when discussing emissions?

Answer: When discussing emissions, it is important to use the appropriate terms, know the context, and present a complete picture. The U.S. Department of Energy (DOE) has a number of tools and resources available to understand and calculate the emissions benefits of alternative fuels and vehicles (see below). But first, let's get back to the basics.

Criteria Pollutants versus Non-Criteria Pollutants
Vehicles emit both criteria pollutants and non-criteria pollutants. In compliance with the Clean Air Act, the U.S. Environmental Protection Agency (EPA) classifies six common pollutants as criteria pollutants based on certain health and environmental standards:
  • Carbon monoxide (CO)
  • Oxides of nitrogen (NOx)
  • Particulate matter (PM)
  • Ozone
  • Oxides of sulfur (SOx)
  • Lead
For more information about criteria pollutant emissions:(

Greenhouse gases (GHGs), including carbon dioxide,are considered non-criteria pollutants. The following also fall into this category:
  • Volatile organic compounds (VOCs)
  • Total hydrocarbons (HCs)
  • Methane
  • Air toxics
  • Other organic gases

For more information about GHG emissions: (

Measuring Emissions
You can evaluate vehicle emissions through a number of lenses. Considering emissions in different contexts can present a more impactful picture, depending on the stakeholder.
  • Life cycle emissions:Emissions generated through all stages of a fuel's life, including raw material extraction, processing, manufacturing, distribution, use, and disposal or recycling. Life cycle emissions are typically considered when evaluating "global pollutants," or pollutants that have an impact regardless of where they are emitted.  For example, GHGs are usually measured on a life cycle basis.
  • Tailpipe emissions: Emissions directly from the exhaust of the vehicle. Tailpipe emissions are considered when looking at "local pollutants," or pollutants that impact air quality directly where they are emitted. For example, criteria pollutants, such as PM, are typically measured as tailpipe emissions.
  • Evaporative emissions: Emissions from the vehicle's fuel system and during the fueling process, not including the combustion of the fuel. Evaporative emissions are also considered when evaluating "local pollutants."
When quantifying or presenting emissions benefits for a particular project, make sure to ask yourself which type of information would have the most impact. For example, an air quality organization (e.g., your local American Lung Association chapter) would like to hear about tailpipe and evaporative emissions. A national company focused on their footprint and impact on climate change would want to hear about life cycle emissions.

Emissions Standards
EPA sets tailpipe and evaporative emissions standards for new vehicles.
  • For information on federal non-GHG emissions standards, including CO, NOx, PM, and organic gases, visit the EPA's Emission Standards Reference Guide: EPA's Tier 3 Vehicle Emission and Fuel Standards Program page ( covers the regulations for light-duty, medium-duty, and some heavy-duty vehicles that will be phased in beginning in 2017.
  • For information about federal GHG emissions standards, which are implemented in conjunction with the National Highway Traffic Safety Administration's fuel economy standards, visit EPA's Regulations & Standards page:

The California Air Resources Board (CARB) enforces vehicle emissions standards for California that are more stringent than federal EPA standards. Vehicles may be certified as compliant with federal standards, CARB standards, or both. For information on CARB's emissions standards, visit the Mobile Source Program Portal ( Several other states have chosen to comply with certain CARB standards as well, so read up on the requirements in your state. See the AFDC Laws & Incentives website for more information (

Other Considerations
It is important to take into account the "full package" when looking at alternative fuel vehicle (AFV) emissions; again, try to anticipate questions from the audience to tease out the most relevant information. For example, keep the following in mind:
  • While a fuel may not offer large reductions in one pollutant, it may offer significant benefits in other pollutants.
  • Emissions information should also be presented in the larger context of federal and state regulations.
  • Be sure you are comparing "apples to apples" when looking at AFV and conventional vehicle emissions. For instance, look at which pollutants are covered, and whether tailpipe, life cycle, and/or evaporative emissions are being measured. Every study is different, so it can be very difficult to compare outcomes of one to outcomes of another.

Emissions Analysis Tools
With all of that in mind, the following tools can be used to calculate fleet emissions and plan for overall emission reductions:

Clean Cities Technical Response Service Team