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An ambitious attempt by U.S. lawmakers to put more self-driving cars on the country’s roads has stalled out in the Senate, where some Democrats are raising new doubts about the technology.

For a few senior party lawmakers, the fear is that these computer-driven vehicles aren’t yet ready for major roadways or might be susceptible to cyber attacks. So they’re standing in the way of a Senate vote on the bill, demanding changes that they say are essential to protect riders’ safety.

Chief among the critics is Sen. Dianne Feinstein, whose state of California is a home base and critical testing ground for companies like Uber, Tesla and Google.

In December, Feinstein sounded off in early opposition to lawmakers’ self-driving car bill. And in an interview Wednesday at the U.S. Capitol, the Democratic lawmaker doubled down — stressing that she is “apprehensive as to whether we’re ready” for a world in which highly autonomous sedans share the road with humans.

“It seems to me that you have to have a period of time where these cars are put on roads, but not necessarily heavily impacted California freeways that are going 65 to 75 miles an hour,” she said. “That’s my view, and I’m a driver, and I know I wouldn’t feel very comfortable.”

In California, though, Google search giant’s self-driving car division, Waymo, racked up roughly 636,000 miles’ worth of rides on local roads just last year. In a sign of the stakes, the company even paid Feinstein a visit in Washington, D.C., this week to try to pitch her on the technology.

“People need to be assured, and they need to be assured over time,” Feinstein told Recode. “And you can’t just dump something on a freeway and have people looking over saying, ‘My God, there’s no driver.’”

Members of Congress first set their sights on autonomous vehicles this spring, beginning in the House. Lawmakers there specifically sought to help tech giants and automakers obtain special exemptions so that they could test droves of new experimental vehicles around the country — without adhering to the same safety standards that apply to older cars. Their bill, called the Self-Drive Act, won swift, broad approval from House Democrats and Republicans alike.

But the Senate has squabbled a bit more over its own proposal, the AV START Act. Since last fall, chamber pols have raised a litany of objections, from the protections afforded to driver data collected by cars to the effects they might have on the trucking industry.

And when architects like Republican Sen. John Thune sought to bring the bill up for a speedy, final vote, some skeptical Democrats and Republicans intervened to hit the brakes, placing official holds on the measure that prevented it from being considered and approved.

Among those expressing skepticism at the time was Sen. Richard Blumenthal, who told Recode in a statement that autonomous-driving technology is still “an emerging and unproven technology.”

“As it stands, this bill does not include enough protections to keep drivers, passengers and pedestrians safe,” he said in December, “but I’m hopeful we can strengthen these safeguards while allowing for limited testing and continued innovation.” His office did not comment this week as to whether the senator remains opposed.

Last month, Massachusetts Sen. Edward Markey similarly raised a formal objection with the bill. And on Thursday, an aide to the Democratic lawmaker said he still has his doubts — and aims to “strengthen provisions in the bill related to automotive defects, cyberattacks, and consumer privacy, especially on the privacy provisions.”

Despite those setbacks, the authors of self-driving car legislation said this week that they’re hopeful. “We’re willing to work with people who have objections, and address their concerns, if it can be done in a way that doesn’t undermine the purpose and the basic framework of the legislation,” Thune told Recode in an interview.

But even he acknowledged that the toughest roadblock of all might be Feinstein.

“I don’t know if she’s asked for anything in particular; she just doesn’t like the bill,” he said of his Democratic colleague on Wednesday.

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