General Motors Co's semi-autonomous 'Super Cruise' system will allow drivers to take their hands off the steering wheel for extended periods, but will stop the vehicle automatically if drivers are not attentive, according to a government letter made public on Monday.

The largest United States (US) automaker in September 2014 unveiled planned technology to allow drivers on highways to let the vehicle take over driving itself.

But if the road has too many twists and turns or the vehicle detects the driver is not paying attention, it issues a series of alerts. If the human driver does not take over, the vehicle will automatically slow down and then put on the hazard lights.

GM, which initially planned to unveil the technology on the CT6 in late 2016, said in January it would not unveil Super Cruise until 2017.

The US National Highway Traffic Safety Administration (NHTSA) said in a letter to GM that it was permissible for the automaker to automatically activate hazard lights if a driver fails to respond to alerts by a self-driving vehicle that has brought itself to a stop, but it raised concerns that the system is safe.

In March, GM asked NHTSA to confirm it was appropriate for the vehicle to activate the hazard lights if it was brought to a stop. NHTSA said in its letter it urged GM to "ensure that this fallback solution does not pose an unreasonable risk to safety."

NHTSA said GM's Super Cruise system will in some situations "alert the driver to resume steering." It said that if the driver is incapacitated, unresponsive or otherwise unwilling to retake the controls, "Super Cruise may determine that the safest thing to do is bring the vehicle slowly to a stop... in or near the roadway."

GM's monitoring system has facial recognition software that can detect if a driver is falling asleep or not paying attention, spokesman Kevin Kelly said Monday. If so, the system issues alerts: a red visual display telling a driver to take control followed by a seat vibration and then a recorded audio message. If drivers ignore all those, GM's OnStar system will communicate with the driver.

GM confirmed details of the Super Cruise system outlined by NHTSA in a separate letter reviewed by Reuters.

Concerns about vehicles that drive themselves for limited periods were raised after the May 7 death of Ohio technology company owner Joshua Brown in a Tesla Motors Inc Model S while the car's semi-automated Autopilot system was engaged. NHTSA is still investigating that crash.

Similar systems due on the market in coming years include Traffic Jam Pilot from Volkswagen AG's Audi unit in 2018.

In September, Tesla updated its semi-autonomous driving system Autopilot with new limits on hands-off driving and other improvements that likely would have prevented a fatality in May, the company said. The updated system will require drivers to respond to audible warnings to take back control of the car.