If the police asked a driver to appear in court in court stating 12 demerit points, the driver is usually being prosecuted for driving at a speed that is 40kph to 50ph higher than the speed limit. E.g., 90kph on a 50kph road
Had you been travelling at 90 to 100 kph on roads without any speed limit signs?
If the driver drives 50kph to 60kph above the speed limit, he gets 18 demerit points. More than 60kph, 24 demerit points.
Once the driver goes more than 40kph above the speed limit, the driver will be prosecuted in court without being offered a composition of fine. He can only make his appeal in court before the judge. The fine for going 40kph for first offender is usually about $200 but can be more depending on circumstances. E.g., if the judge believes the offender is wasting everyone's time by denying the charges despite overwhelming evidence that state otherwise, he can order the offender to pay the top limit of the fine ($2,000--I think).
By the way, a composition of fine is something like an offer made by the government to the accused to pay to avoid inconvenience of prosecuting minor offences. An offender may choose to appeal and/or refuse the offer of composition, the agency who made the offer may then choose to accept the appeal or prosecute the offender in court.
Travelling at more than 40kph above the speed limit is dangerous driving, and is a serious offence which the offender cannot be given an offer to composite the fine. The offender must be prosecuted in court.
Claiming ignorance of speed limit is usually not good enough material in court. Singapore like most countries in the world has a default speed limit when there are no posted speed limit. Singapore's default limit is 50kph.