Tesla has launched a software update for its vehicles - enabling the cars to have an "autopilot" mode.

While not fully self-driving, the software means the Model S and new Model X can "automatically steer down the highway, change lanes, and adjust speed in response to traffic".

Tesla chief executive Elon Musk said users adopting the software - available in North America from Thursday - should exercise caution while using it.

Speaking in October 2014 at the launch of Dual Motor Model S and Autopilot, Mr Musk explained how the system worked.