The doors can be set to lock automatically when driving at a speed above 6mph (10km/hr):
To activate
With the ignition on (not started) - press and hold the dash button marked with a padlock.
You should hear a bleep and a message on the multifunction screen.
To deactivate
With the ignition on (not started) - press and hold the dash button marked with a padlock.
You should hear a bleep.