Motorcoach operators could see significant improvements in vehicle reliability and lower operating costs under a proposed rule announced by the US Environmental Protection Agency (EPA) that would overhaul portions of federal heavy-duty diesel emissions requirements.

The proposal, unveiled by EPA Administrator Lee Zeldin, would revise elements of the agency's 2022 heavy-duty nitrogen oxide (NOx) emissions rule while retaining the underlying emissions standards. Among the most significant changes for the bus industry is a proposal to eliminate emissions-related engine derates — automatic speed restrictions triggered when diesel exhaust fluid (DEF) systems malfunction.

For motorcoach companies, where unexpected roadside breakdowns can strand passengers and disrupt schedules, the proposed change addresses a long-standing operational concern.

Instead of forcing vehicles into reduced-speed mode or shutting them down due to DEF system failures, newly manufactured buses and other heavy-duty diesel vehicles would continue operating while providing drivers with audible or visual alerts, allowing repairs to be made safely, as METRO previously reported.

The EPA estimates that the broader package of regulatory changes could save the commercial vehicle industry approximately $12 billion, including up to $6,000 per new vehicle, by reducing warranty-related compliance costs and providing manufacturers with additional flexibility to meet emissions requirements.

The agency also proposes extending implementation timelines for portions of the 2027 heavy-duty emissions rule, allowing manufacturers additional time to validate new emissions technologies before bringing vehicles to market.

The American Bus Association (ABA), which has advocated for changes to DEF-related requirements, said the proposal addresses issues that directly affect passenger transportation.

"ABA welcomes EPA's announcement proposing further action to address DEF-related deratements that can create serious safety, reliability, and operational challenges for motorcoach operators," said ABA CEO Fred Ferguson. "ABA has consistently supported reforms that reduce unnecessary and sensor-related derates, improve equipment reliability, and allow operators to address emissions-system issues safely without stranding passengers or creating roadside hazards."

Motorcoach operators have increasingly reported that sensor failures — not necessarily emissions-system failures — can trigger engine derates, forcing coaches to slow dramatically or become inoperable even when passengers are on board.

Phil Streif, co-owner and vice president of Vandalia Bus Lines in Caseyville, Illinois, called the proposal "a tremendous step toward restoring common sense to emissions policy."

"Eliminating vehicle derates addresses a long-standing safety concern for commercial transportation while still allowing the industry to meet meaningful emissions standards," Streif said.

He credited transportation companies and industry organizations for working together over several years to bring operational concerns to regulators.

"We appreciate the administration and the dedicated professionals within the EPA who took the time to listen to our concerns, engage in constructive dialogue, and carefully consider the real-world impact these regulations have on businesses, drivers, and the traveling public," Streif said.

While industry groups praised the proposal, clean transportation advocates expressed concern that revising warranty, useful-life, and enforcement provisions could weaken long-term emissions performance.

CALSTART, a nonprofit focused on advancing clean transportation technologies, said maintaining the underlying NOx emissions standards is positive but warned that relaxing implementation requirements could create uncertainty for manufacturers and reduce emissions benefits over time.

"EPA's decision to retain the current stringency of the NOx emissions program for heavy-duty vehicles is a strong testament to the hard work the agency undertook in 2022," said CALSTART President/CEO Michael Berube. "Unfortunately, EPA's latest proposed changes risk introducing confusion and additional market uncertainty for the heavy-duty truck and engine manufacturers and the fleets that depend on reliable vehicle operation."

Berube urged EPA to work with manufacturers and fleet operators to address reliability concerns while preserving long-term emissions reductions.

The EPA will accept public comments on the proposal for 45 days and will hold a public hearing before issuing a final rule.

If adopted, the changes would affect newly manufactured heavy-duty diesel vehicles, including motorcoaches. At the same time, the agency also seeks comment on developing guidance that could allow similar relief for existing vehicles already in service.

For motorcoach operators, the proposal represents a potential shift toward emissions regulations that prioritize both environmental performance and operational reliability. This issue has become increasingly important as fleets continue investing in newer, cleaner diesel equipment while maintaining dependable service for passengers.

The EPA is proposing to remove DEF-related engine derates for new heavy-duty diesel vehicles.