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How Heavy is Too Heavy? Idle Reduction Equipment Impacts Weight Limit Restrictions

April 20, 2015

Photo of a red semi-truck driving on a highway surrounded by forest.

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

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 State Recognition of the Auxiliary Power Weight Exemption to Gross Vehicle Weight is available from the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE). As the map shows, 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.

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 EERE's National Idling Reduction Network News and Argonne National Laboratory's Idle Reduction Tools and Outreach Materials for more information on idle reduction technologies and state vehicle weight limit exemptions for this equipment.

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