A driverless car bill that had been speeding through the Senate hit a speed bump in the upper chamber this week.

The measure was set up under a fast-track process with the hopes of getting it over the finish line this year, but multiple senators are blocking the legislation from moving forward.

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Bill sponsors hope to iron out some of the kinks over the next few weeks, but it's unclear whether major sticking points, such as concerns over safety and trucks being excluded from the legislation, can be easily resolved. The Senate is also facing a tight timeline and daunting to-do list before the end of this year, including spending bills and a major tax overhaul.

“Some folks have some holds on it. There are some folks who have some issues,” Sen. Gary Peters Gary Charles PetersBiden's six best bets in 2016 Trump states GOP set to release controversial Biden report Democrats fear Russia interference could spoil bid to retake Senate MORE (D-Mich.), one of the bill’s sponsors, told The Hill. “We’re going to keep working on it.”

The measure, which would remove obstacles for getting more self-driving cars on the road, is currently being “hotlined” in the Senate — a process that allows bill sponsors to informally identify any potential concerns with the legislation and gauge whether the proposal could pass the chamber unanimously.

The goal of the legislation is to help the car industry speed up the deployment and testing of autonomous vehicles by gradually waiving traditional automobile standards — like steering wheels and brake pedals — for up to 80,000 vehicles.

Under the bill, the Department of Transportation would have the authority over for setting industry safety standards, pre-empting a patchwork of varying state laws. State and local authorities will still continue to govern traffic safety, vehicle registration and law enforcement issues.

Congress has been racing to address autonomous vehicle technology with automakers pledging aggressive timelines to bring self-driving cars to the market. There are currently no federal safety standards specifically governing driverless vehicle technology, including software and sensors.

Across the Capitol, the House already passed its own driverless car legislation and has been awaiting action in the Senate.

But there are multiple lawmakers who have put a hold on the Senate’s bill, including Sens. Ed Markey Edward (Ed) John MarkeySchumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Democrats see fundraising spike following Ginsburg death Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy MORE (D-Mass.) and Richard Blumenthal (D-Conn.), according to Bloomberg.

Markey wants to strengthen provisions regarding automotive defects, cyberattacks and consumer privacy, his office said. And Blumenthal has expressed safety concerns with the legislation.

Consumer advocates worry that the bill’s exemption language is too broad and would allow manufacturers to opt out of safety features like air bags. They also worry that the legislation will allow manufacturers to rush to market before the technology is ready.

During the committee markup, Blumenthal offered and withdrew, an amendment that would have required a driver to be behind the wheel of an autonomous vehicle, even for the most advanced cars that are supposed to be fully autonomous.

But bill sponsors, including Senate Commerce Committee Chairman John Thune John Randolph ThuneGraham: GOP will confirm Trump's Supreme Court nominee before the election Tumultuous court battle upends fight for Senate What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-S.D.), say that provision would have undermined their entire effort.

Other holds on the measure could be in place simply to give some lawmakers more time to read through the bill and understand its impact.

There could also be lingering concerns over trucking being left out of the bill — a far trickier issue to deal with.

One of the biggest points of contention during debate over the bill was whether to include trucks and other large commercial motor vehicles in the measure.

Trucking is one of the primary industries that is expected to be transformed by automated vehicles, with companies like Uber already jumping into the long-haul driverless trucking space.

But trucking unions are concerned about the effect that disrupting the status quo would have on employment, while industry groups argue that cars and trucks should be covered by the same framework.

Thune and other Republicans such as Sen. James Inhofe (R-Okla.), who says he is not one of the lawmakers blocking the bill, had pushed to include trucking in the Senate measure. Peters and Democrats wanted to maintain the status quo.

Ultimately, Peters won out. The House’s driverless car measure also does not include trucks, due to jurisdictional issues with the committee that wrote the bill.

But there could be legislative changes made to the Senate measure before it is brought to the floor, Peters told The Hill, though it’s unclear what those tweaks would be.

"There are a variety of things involved, which is just natural. It’s nothing we didn’t anticipate and we’re working through some of the issues,” he said. “But the fact that it is hotlined is encouraging. Very encouraging.”