The sponsors of a Senate bill to set the federal framework for driverless vehicles said Wednesday they were making progress on their effort to pass it by unanimous consent. But they resisted calls to amend the measure and said they may pursue a floor vote instead.

After a Senate Commerce, Science and Transportation Committee field hearing at the Washington Auto Show, Chairman John Thune told reporters he will continue to work through safety concerns that three Democratic senators had over the bill. But he said he wouldn’t cave to demands that would undermine the purpose of the legislation.

Democratic Sens. Richard Blumenthal of Connecticut, Edward J. Markey of Massachusetts and Dianne Feinstein of California have holds on the bill, preventing unanimous consent, Thune said. The South Dakota Republican said he was working with committee members Blumenthal and Markey to address their concerns about safeguards in the event that technology fails, but he indicated he wouldn’t be open to altering the bill significantly.

“For one, they want a fallback. They basically want a driver,” Thune told reporters. “Well, these are driverless vehicles. It seems to kind of defeat the purpose. … There are things they are asking for that I think could inhibit not only the innovation, the development of these technologies and their deployment. So I don’t know. We’re trying to work with them but some of the things they’re asking for are kind of challenging.”

The bill’s co-sponsor, Michigan Democrat Gary Peters, said he was working through the issues with his fellow Democrats.

“A big part of this issue is making sure people understand what the bill involves,” Peters told reporters.

Blumenthal said during the hearing he would likely vote for “a bill” on autonomous vehicles but seemed to be holding out for more changes to the measure the committee approved by voice vote in October. In the meantime, he said he would push for technology like crash avoidance to be mandatory in all vehicles. Crash avoidance technology encompasses things such as automatic braking and lane assistance.

He said he was seeking to prevent “another generation of cars that may be unsafe at any speed” and said the example of malfunctioning ignition switches showed that even the simplest technology in cars can present safety challenges.

“Unsafe at Any Speed” was the title of Ralph Nader’s 1965 book that accused manufacturers of failing to make cars safe. The federal government established the National Highway Traffic Safety Administration in the wake of the book’s publication. 

Thune said addressing Feinstein’s concerns was more challenging because they were less specific. She had concerns about the overall safety of the technology, he said.

If the sponsors are unable to gain unanimous consent to get the bill through the Senate, they said they’d hope to get floor time for a roll call vote. Thune and Peters predicted the bill would receive at least 80 votes in such a scenario.  

The House passed its own bill on autonomous vehicles by voice vote in September.