About 50 supporters have turned up at the Supreme Court in Samoa for the start of the legal challenge to the government's planned driving side switch.
In three weeks, motorists in Samoa will have to drive on the left side of the road instead of the right.
But the lobby group Pass is arguing the change is unconstitutional on safety grounds.
Our correspondent in Apia, Autagavaia Tipi Autagavaia, says the actual hearing is expected to be quite brief:
"They have around six witnesses including this witness from New Zealand, his name ie Graham John Williams. He is a former police officer investigating traffic accidents in New Zealand but now running a privately owned investigation consultancy. It is scheduled for this case to take about two or three days."
Autagavaia Tipi Autagavaia says the judge has previously stated he will reach a decision before the change comes into force in early September