SBS Transit has earned $2,040,000 for improvements to 25 bus services, while SMRT Buses received $1,403,300 for improvements to 20 services.

The incentives were given out as part of a trial "carrot-and-stick" scheme to get public bus operators to improve service reliability and reduce bus bunching.

The money was given for improving the regularity of 45 bus services in all, said Transport Minister Khaw Boon Wan on Tuesday, in a written reply to a query about bus reliability filed by MP Liang Eng Hwa.

Mr Khaw said there was a 20 per cent reduction in average additional wait times for the bus services, from December 2015 to August last year.

This is the final assessment period of the Bus Service Reliability Framework (BSRF) pilot, which started in February 2014. From the previous three assessment periods, SBS received about $2.87 million in total, while SMRT got $1.23 million.

Under the BSRF, public bus operators are given monetary incentives for reducing commuters' excess waiting time, or handed penalties if the waiting time increases.

The framework also uses an "On Time Adherence" standard, which measures the proportion of times that the bus service arrives less than two minutes earlier, or five minutes later than its scheduled timing.

The BSRF is incorporated into the bus contracting model, which the bus industry fully transited to last September, and has been implemented for 108 services, resulting in a 15 per cent reduction in average additional wait times, said Mr Khaw.

National University of Singapore transport researcher Lee Der Horng said it was appropriate to raise the BSRF standards to reflect the changing operating environment, which includes an expansion of the bus lane scheme.

From March 21 last year, the Land Transport Authority extended the operating hours of full-day bus lanes by three hours, until 11pm.