The battle over car rules is a math problem, and it might have life-or-death consequences. At issue is how President Donald Trump’s administration will estimate potential fatalities in new cars that meet stringent standards on fuel efficiency established under former President Obama.

Corporate average fuel economy opponents have been shifting their criticism after research had thrown cold water on their first line of attack: “down-weighting.”

David Greene, research professor in civil and environmental engineering, recently talked to Science about the controversial issue saying the rationale has been debunked.

“The problem with that argument is that it didn’t take into account that all of the light-duty vehicles would be made lighter and the cars weren’t made smaller,” he said. Lighter cars will lead to less kinetic energy involved in any crash and the force between the vehicles will be reduced.

Trump’s Effort to Roll Back Auto Efficiency Rules Could Hinge on Debate Over Safety