The NHTSA has asked the company to consider if stopping the vehicle on the road poses a safety risk.
The National Highway Traffic Safety Administration has raised concerns over General Motors' Super Cruise system.
GM's planned implementation of semi-autonomous technology will automatically slow the car to a stop if the driver fails to retake control if necessary. In a letter spotted by The Wall Street Journal, the NHTSA asked GM to review the potential safety threat of automatically stopping a car "in or near" the roadway.
"We urge GM to fully consider the likely operation of the system it is contemplating and ensure that this fallback solution does not present an unreasonable risk to safety," wrote NHTSA chief counsel Paul Hemmersbaugh.
Following Tesla's first fatal Autopilot accident, the company updated its semi-autonomous technology to discourage misuse. Consumer Reports criticized the technology for allowing drivers to leave their hands off the wheel "for minutes at a time." The system will automatically disable auto-steer in certain situations, but it simply prompts the driver to retake control rather than autonomously stopping the vehicle.
The NHTSA's letter to GM appears to highlight the lingering ambiguity in automotive safety regulations for autonomous cars. There is no clear and complete guidance detailing exactly how a vehicle should react if the human driver is not paying attention, the road conditions exceed the system's abilities, or if a sensor fails.
A GM spokesperson defended the Super Cruise approach, pointing out that the car will only stop itself in a "worst-case scenario" after the driver has received visual and verbal warnings.
"We're not going to put any system into the marketplace unless we know it's safe," the spokesperson added.