Tesla will update its cars' Autopilot function with new safety features designed to prevent collisions, the company has announced.

Chief executive Elon Musk told reporters on Sunday that his cars would soon make greater use of the on-board radar to detect obstacles ahead.

The car will also do more to make sure drivers are paying attention while in self-driving mode.

A Tesla driver died earlier this year when the technology missed a lorry.

As part of the update to self-driving mode, if repeated warnings to hold the steering wheel are ignored, the vehicle will need to be parked before the autonomous functions can be re-engaged.

When drivers activate Autopilot, the car takes control - keeping pace with traffic and even changing lanes. The 200-plus additions to the software come as it is being investigated by the US road safety regulator.

A Tesla driver was killed in May when the car he was driving hit a lorry that had been turning left - Autopilot was unable to spot the white trailer against the bright sky.

No date has been announced for the conclusion of the investigation, the US National Highway Traffic Safety Administration told the BBC.

In a call with reporters, Mr Musk said the NHTSA appeared to be “happy” with the Autopilot update.

'Aggressive'

Mr Musk will hope that the update will appease those critical of Tesla's introduction of the self-driving technology.

In July, a consumer rights group in the US accused Tesla of an “aggressive rollout of self-driving technology”. It urged Tesla to rename the technology as it felt “autopilot” was “misleading and dangerous”.

The updates to Autopilot, Mr Musk said, will give the car the ability to make greater use of the radar to spot potential dangers.

"After careful consideration, we now believe it can be used as a primary control sensor without requiring the camera to confirm visual image recognition,” Mr Musk wrote in a blog post.

The challenge in using radar is in avoiding “false alarms”, he added, where small objects like cans could be interpreted as a threat.

Overhead signs or bridges can also be misinterpreted if the road dips. To combat this, Tesla cars are going to be used to “learn” about the road.

"Initially, the vehicle fleet will take no action except to note the position of road signs, bridges and other stationary objects, mapping the world according to radar,” Mr Musk wrote.

"The car computer will then silently compare when it would have braked to the driver action and upload that to the Tesla database.

"If several cars drive safely past a given radar object, whether Autopilot is turned on or off, then that object is added to the geocoded whitelist.”

Tesla did not give a date for the Autopilot update to be rolled out. The radar hardware has been a part of the company’s cars since October 2014.