If a car owner receives payment from those who car pool with him, is he operating a pirate taxi service? Or is he merely in an arrangement similar to when people split the bill for a meal?
The Land Transport Authority (LTA) is expected to come up with an answer as an increasing number pay for car pooling services, as an alternative to public transport and car ownership.
A number of car pooling sites have already sprouted. Among the latest and most prominent is ShareTransport.sg, which has been pairing car-poolers with car owners for the past 20 months.
It says it has 15,000 users now, and expects to exceed 20,000 next year.
The LTA has been keeping an eye on the development for about two years now, but has not come out with rules on payment.
"LTA is supportive of car pooling as it provides greater transport choices to the commuting public. We are working on guidelines to mitigate any potential abuse of car pooling as illegal taxi services," a spokesman said.
Mr Moh Hon Meng, 44, who started ShareTransport.sg in March last year, said simple rules might include requiring that drivers have full-time jobs, and not rely on car pooling payments as their main source of income.
Also, rides must be "on the way" - meaning, if car owners live in Hougang and work in Shenton Way, they cannot drop someone off in Jurong.
And charges should be based on a "cost recovery" principle and not a profit-making one.
"Even if we have altruistic drivers who are all willing to give free rides, passengers would not want to take them, because they will feel bad. The passengers actually want to pay their share," said technopreneur Moh, who co-founded online unit trust distributor Fundsupermart.com more than 10 years ago.
A driver stands to recoup up to $600 a month in costs by offering car pooling services, he added.
ShareTransport.sg user Kimberley Ang, 46, said she used the service for about a year.
"I paid $5 for a ride from my home in Sengkang to PSA Building in Alexandra Road. A cab ride would cost $26," she said.
"It's convenient, it's safe, and I made friends along the way."
The marketing manager however, has stopped car pooling since changing jobs recently.
There is no match yet for a driver going from Sengkang to Pandan Loop, her new place of work. She put that down to not enough people knowing about car pooling.
"It's a great idea, but there's not enough awareness yet. Which is strange, because in the old days, people used to car pool to get into the CBD," she said, referring to the time when cars with four occupants or more could enter the city centre without paying for an area-licensing coupon.
That was dropped in 1994, when drivers began picking up passengers from bus stops.
On the flipside, some observers believe taxi drivers are likely to complain if car pooling becomes more prevalent.
"For obvious reasons, this will not go down well with cabbies," said veteran taxi driver Tony Pang, 63.
Another cabby Victor Yow, 67, however, said car pools will not be a threat.
"They are likely to operate during peak hours, when there are more commuters than taxis can cater to."
Still, car-poolers feel the LTA should come out with clear, enforceable rules to separate those who share rides from those who run illegal cab operations.
Said Mr Moh, whose site also lets people share a cab or even a bus: "It should be clear what car pooling is, and what providing a transport service is."
Car pooling is catching on in a big way in America, Europe and parts of Asia.
Mr Brian Hsu, a 32-year-old New Zealand-based Taiwanese who runs a software business in China, hosts Carpool King, which offers car pool listings in Singapore and a few other places in the region.
Mr Hsu said: "I started Carpool King in 2007. I was commuting and got stuck in traffic jams every day. When I looked around, most vehicles were single-occupant ones like mine. So I started this service for financial, societal and environmental benefits."
Of the 15,000-plus registered Carpool King users, nearly 5,000 are in Singapore.