The government declared a two-day holiday and three-day ban on liquor sales while people get used to the change.
The country's Chamber of Commerce has estimated the switch would cost up to $500 million to initiate.
The government hopes Samoans with relatives in New Zealand and Australia will ship cars to the islands for much less money than islanders can get them from America.
Thousands protested on the streets and presented a legal challenge to halt the switch which was overturned.
The change has enraged the operators of Samoa's colourful fleet of buses, with One News reporting only 20 of the country's buses have been switched to be left-hand-side driving compliant.
"We don't know what to do, we have no hope, no assets, we've got big loans," bus owner Nanai Tawan said.
Bus companies have said it costs around $12,000 per bus to change the doors so people are not getting out in the middle of the road.
However, Minister of Transport Tuisugaletaua Aveau said the owners' figure was exaggerated.
"We have already had people who changed the side...they came up with the figure of $1500," Mr Aveau said.
Samoa was the first country to switch road direction for 40 years. The changes took effect at 6am today, Samoan Time.
