The AV START Act, which would allow automakers to sell more than 80,000 autonomous cars each per year, is facing an uncertain future on Capitol Hill, as at least five senators have expressed serious concerns about it, reports the Detroit News.

The setback for the bill, which aims to create national self-driving standards, comes on the heels of a pair of major crashes involving Tesla and Uber self-driving cars, one of which killed a pedestrian.

While a similar measure has already passed in the House, key backers of the bill, including Sens. Gary Peters (D-MI) and John Thune (R-SD) are still attempting to gain the support of reluctant senators, including Dianne Feinstein (D-CA), Richard Blumenthal (D-CT), Ed Markey (D-MA), Kirsten Gillibrand (D-NY), and Tom Udall (D-NM).

Those five senators say the bill indefinitely preempts state and local safety regulations — even if federal safety standards are never developed, according to the news report.

Under the proposed bill, the Department of Transportation would be responsible for producing a report on provisions in federal motor vehicle safety standards that need to be updated to ensure self-driving cars can perform tasks that are currently required of human operators as well as creating a technical safety committee for highly automated cars, noted the Detroit News report.

The bill would prohibit states and other local jurisdictions from adopting regulations related to car design, construction, software or communication. States would be allowed to regulate registration, licensing, liability, education and training, insurance, and traffic laws.