The US Department of Transportation last week closed the public comment period on a plan that overturns the previous administration's greenhouse gas performance measurement rule. This regulation was originally designed to shift the focus of clean air rules away from pollutants and toward reducing carbon dioxide emissions, all in the name of fighting global warming.

Under the 2012 transportation law known as MAP-21, Congress ordered the Federal Highway Administration to come up with performance measures to deal with "traffic congestion and on-road mobile source emissions."

Five months before last year's election, the Obama administration issued a rule interpreting this particular line of text to mean that the could force states to implement a "CO2 emissions measure."

Currently, federal rules require that state transportation officials keep statistics on accidents, pavement condition, traffic volume, congestion and pollutants like ozone, carbon monoxide and particulate matter. Instead of going after "dirty" air containing those pollutants, the previous administration's rule would require states to track carbon dioxide using an estimate, rather than a direct measurement.

Each state was to take general figures on the volume on fuel sales and multiply them by an estimate of the number of vehicle miles traveled statewide to generate the estimate of carbon dioxide output. While the cost of implementing the previous administration's rule was $11 million, its purpose, as underscored by Democratic legislators arguing to preserve the rule, was to shift governmental priorities toward the fight against global warming. Twenty-three senators and twenty-seven representatives from the minority party urged the department to preserve the rule because it created national carbon dioxide emission goals for the transportation sector.

"Repealing the greenhouse gas measure inhibits the ability of decisionmakers to properly make progress toward this national goal," the lawmakers wrote in a November 8 letter. "A greenhouse gas performance measure is critical for state departments of transportation and metropolitan planning organizations to accurately determine the type of investments needed. Without this measure, state departments of transportation and metropolitan planning organizations will be planning based on an incomplete picture of the system."

A coalition of manufacturers countered that these lawmakers lost the battle in Congress, and that the federal agency lacks the legal authority to re-write the law in this way.

"Congress had a chance to include greenhouse gas related measures in MAP-21 when it was deliberated in both the House and Senate and chose not to do so," American Road and Transportation Builders Association CEO T. Peter Ruane wrote in a November 15 letter.

The Federal Highway Administration now says the prior administration's rule was "duplicative" and "burdensome." The agency also questioned the reliability of using sales receipts and traffic volume as a proxy for emissions at the tailpipe that can vary with speeds and operating conditions. After it takes into account the public comments, the agency can publish a final regulation undoing the Obama-era rule.