The National Highway Traffic Safety Administration is studying whether older-model electrified vehicles should have pedestrian-warning sounds that later models have standard by law.

Such vehicles make little noise and can therefore be hard for pedestrians to hear. The 2018 Federal Motor Vehicle Safety Standard law, aimed at protecting blind and visually impaired pedestrians, requires the audible warning systems in hybrid and fully electric vehicles under 10,000 pounds starting with models made in March 2021.

The study will consider whether models as far back as 1997 should have the audible-warning technology. Based on the findings, vehicles made before March 2021 could be required to have the technology.

The NHTSA estimates there are 9.1 million hybrid and electric vehicles that would need to be retrofitted with the technology if it finds the older models should have it.

Prompting the study was a petition filed last July requesting that the federal law apply to all hybrid and fully electric vehicles.