A motorist has died in South Australia after a car left the road and caught fire.

The death on Croser Road at Millicent, south-east of Adelaide, is the second in the state since the holiday road period started on December 23.

On Monday, a 67-year-old woman died when her car veered off the road and smashed into a tree south of Adelaide.

She was the only occupant of the car and died at the scene.

Police have urged motorists to adhere to speed limits, take rest stops on long journeys, wear seatbelts and not drink and drive.

* The national road toll period runs from 0001 December 23, 2012, until 2359 January 3, 2013, local times, in line with the Australia New Zealand Policing Advisory Board.