The population of red-plated off-peak cars has fallen by nearly a fifth in the last four years as the increased costs of owning a car have made them less of a draw.
Land Transport Authority (LTA) figures showed that there were 40,438 off-peak cars as at June this year, from 50,040 in December 2010.
Only 61 new off-peak cars were registered in the first six months of this year, down from 81 in the same period last year and 304 in 2012.
Experts said the scheme, which offers rebates but restricts cars from the road between 7am and 7pm on weekdays, has to be revised to be attractive.
Currently, a buyer can get a rebate of up to $17,000 on a new off-peak car, and a $500 discount on annual road tax. Those who convert their normal cars can get a cash rebate of up to $1,100 every six months. An electronic day licence to drive during restricted hours costs $20.
Dr Park Byung Joon, who teaches urban transport management at SIM University, said people do not think the scheme is worthwhile, given the restrictions on usage and high certificate of entitlement (COE) premiums that are now hovering above $60,000.
COE prices used to be around $20,000 in January 2010.
Said transport academic Lee Der Horng from the National University of Singapore: "Since COEs became more expensive, there has been a very clear trend that this kind of price is beyond what those who would use off-peak cars can afford."
Dr Park believes the amount of rebates offered should be reviewed, along with the restricted hours, so that the scheme is more attractive.
Roads are most congested between 7am and 9am, and 5pm and 9pm, so that is when restrictions should be in place, he said.
Prof Lee reckons that the scheme is becoming irrelevant.
He noted that it was meant to limit usage and still fulfil Singaporeans' car-owning dreams, which somewhat contradicts the policy of encouraging people to take public transport.
With options such as car sharing, car rental and taxis available, it may not be that necessary to have a weekend car, he said.
Manager Adam Tan, 47, hopes the restricted hours can be extended to 7.30am so parents have more time to ferry their children to school.
He bought an off-peak car with a COE of $13,000 five years ago, but said he would not have done so if prices were at today's level. "It's not worth it."