On January 18, 2001, the U.S. Environmental Protection Agency (EPA) set current emission standards for criteria air pollutants and their precursors, including nitrogen oxide (NO x ) and particulate matter (PM). Further, on October 25, 2016, EPA and the National Highway Traffic Safety Administration (NHTSA) jointly published the second (current) phase of greenhouse gas (GHG) emissions and fuel efficiency standards for medium- and heavy-duty engines and vehicles (Phase 2). The Phase 2 requirements set emission standards for commercial long-haul tractor-trailers, vocational vehicles, and heavy-duty pickup trucks and vans , and they phase in between model year (MY) 2018 and MY 2027. Glider Kits, Glider Engines, and Glider Vehicles

A glider kit is a tractor chassis with a frame, front axle, interior and exterior cab, and brakes (see Figure 1 ). It becomes a glider vehicle when an engine, transmission, and rear axle are added. Engines are often salvaged from earlier model year vehicles, remanufactured, and installed in glider kits. The final manufacturer of the glider vehicle (i.e., the entity that assembles the parts) is typically a different entity than the original manufacturer of the glider kit. Glider kits and glider vehicles are produced arguably for purposes such as allowing the reuse of relatively new powertrains from damaged vehicles. In short, EPA argued that "it is reasonable to require engines placed in newly-assembled vehicles to meet the same standards as all other engines in new motor vehicles." Current EPA emission standards for NO x and PM (which began in 2007 and took full effect in 2010) are at least 90% lower than previous standards (see Table 1 ). Additionally, based on prior standards, EPA estimated that NO x and PM emissions of any glider vehicles using pre-2002 engines (i.e., before exhaust aftertreatment requirements) could be 20-40 times higher than those of current engines. These emission impacts are compounded by the recent increase in sales of glider vehicles. Estimates provided to EPA during the Phase 2 rulemaking indicate that production of glider vehicles increased by an order of magnitude since 2006—from a few hundred each year to several thousands. Finally, the petitioners argued that the Phase 2 rule had failed to consider the significant environmental benefits that glider vehicles create. On November 16, 2017, EPA (under Administrator Scott Pruitt) proposed a repeal of the emission standards and other requirements on heavy-duty glider vehicles, glider engines, and glider kits based on a proposed interpretation of the CAA. Status of the Proposal

EPA's proposed "Repeal of Emission Requirements for Glider Vehicles, Glider Engines, and Glider Kits" has not been finalized. Some commentators have supported EPA's efforts to reverse the standards and provide relief to the affected glider vehicle assembler industry. They have reiterated the arguments, as presented in the petitions for reconsideration, that EPA lacks the authority to regulate glider vehicles under the CAA because they could not be considered "new motor vehicles." They assert that the benefit of a glider vehicle over a new truck is a more affordable, reliable, and fuel efficient vehicle for purchasing that requires less maintenance, yields less downtime, and yet offers a range of currently available safety features and amenities. On July 26, 2018, EPA withdrew the no-action assurance. However, the memo states that the agency would "continue to move as expeditiously as possible on a regulatory revision regarding the requirements that apply to the introduction of glider vehicles into commerce to the extent consistent with statutory requirements and due consideration of air quality impacts."
On October 25, 2016, the U.S. Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration jointly published the second phase of greenhouse gas (GHG) emissions and fuel efficiency standards for medium- and heavy-duty vehicles and engines. The rule affects commercial long-haul tractor-trailers, vocational vehicles, and heavy-duty pickup trucks and vans. It phases in between model years 2018 and 2027.
Under the rulemaking, EPA proposed a number of changes and clarifications for standards respecting "glider kits" and "glider vehicles." A glider kit is a chassis for a tractor-trailer with a frame, front axle, interior and exterior cab, and brakes. It becomes a glider vehicle when an engine, transmission, and rear axle are added. Engines are often salvaged from earlier model year vehicles, remanufactured, and installed in the glider kit. The final manufacturer of the glider vehicle (i.e., the entity that assembles the parts) is typically a different entity than the original manufacturer of the glider kit. Glider kits and glider vehicles are produced arguably for purposes such as allowing the reuse of relatively new powertrains from damaged vehicles.
The Phase 2 rule contains GHG and criteria air pollution emission standards for glider vehicles. The rule sets limits for glider vehicles similar to those for new trucks, with some exemptions. Under the rulemaking, EPA and various commentators argued that glider vehicles should be considered "new motor vehicles" under the Clean Air Act (CAA) because of recent changes in the glider market. That is, in the decade leading up to the rulemaking, sales of glider vehicles increased by an order of magnitude—from several hundred annually to several thousand or more. EPA and various commentators interpreted this change to be more than an attempt to replace damaged chassis, seeing it instead as an attempt by glider vehicle assemblers to circumvent various federal regulations. At the time, the older model year engines being used in glider vehicles were not required to meet current EPA emission standards for nitrogen oxide and particulate matter (which began in 2007 and took full effect in 2010). Under the Phase 2 rulemaking, EPA estimated that NOx and PM emissions from glider vehicles using pre-2002 engines (prior to exhaust aftertreatment requirements) could be 20-40 times higher than current engines.
Subsequent to the Phase 2 rulemaking, EPA received petitions for reconsideration for, among other provisions, the glider requirements. The petitioners argued that EPA lacks the authority to regulate glider vehicles under the CAA because they could not be considered "new motor vehicles." The petitioners asserted that the benefit of a glider vehicle over a new truck is a more affordable, reliable, and fuel efficient vehicle for purchasing that requires less maintenance, yields less downtime, and yet offers a range of currently available safety features and amenities. On November 16, 2017, EPA (under Administrator Scott Pruitt) proposed to repeal the emission standards and other requirements for heavy-duty glider vehicles, glider engines, and glider kits. On July 26, 2018, EPA (under acting Administrator Andrew Wheeler) stated that it would "move as expeditiously as possible on a regulatory revision regarding the requirements that apply to the introduction of glider vehicles into commerce to the extent consistent with statutory requirements and due consideration of air quality impacts." A rule has not been finalized.
Some in Congress have supported the Trump Administration's efforts to reverse the standards and provide relief to the affected glider vehicle assembler industry. However, EPA's efforts to delay and repeal the rule have prompted criticism from some trucking industry officials, state air agencies, environmentalists, and other lawmakers who fear that increasing production of glider vehicles could result in a fractured vehicle market and significantly higher in-use emissions of air pollutants associated with a host of adverse human health effects, including premature mortality.