- 1 Service 21 DD & 1 Service 21 SD will perform one (1) down-route trip as Service 21 from Changi Road (Bus Stop 82069 - Aft Lor 106 Changi) towards Saint Michael's Terminal before Weekdays AM Peak, departing at 0555 hours & 0610 hours respectively.
- 2 Service 21 DDs & 2 other Service 21 buses will perform down-route trips as Service 21 from Sims Avenue (Bus Stop 82061 - Eunos Stn/Int) towards Pasir Ris Interchange before & during Weekdays AM Peak, departing at 0550 hours, 0600 hours, 0618 hours & 0631 hours.
- 2 Service 21 buses will perform one (1) down-route trip as Service 21 from Changi Road (Bus Stop 82069 - Aft Lor 106 Changi) towards Saint Michael's Terminal before Saturdays AM Peak, departing at 0555 hours & 0606 hours respectively.
- 1 Service 21 bus will perform one (1) down-route trip as Service 21 from Changi Road (Bus Stop 82069 - Aft Lor 106 Changi) towards Saint Michael's Terminal before Sundays & Public Holidays AM Peak, departing at 0555 hours.
- 1 AMDEP Service 21 DD will
- perform two (2) supplementary trips as Short Trip 21A during Sundays & Public Holidays AM Off-Peak & PM Peak, departing Geylang Road (Bus Stop 80089 - Bef Lor 18 Geylang) at 1652 hours & 1720 hours.
- perform one (1) down-route trip as Service 21 from Geylang Road (Bus Stop 80089 - Bef Lor 18 Geylang) towards Saint Michael's Terminal during Sundays & Public Holidays PM Peak, departing at 1751 hours.
- 1 AMDEP Service 21 DD will
- perform two (2) supplementary trips as Short Trip 21A during Sundays & Public Holidays PM Peak, departing Geylang Road (Bus Stop 80089 - Bef Lor 18 Geylang) at 1715 hours & 1744 hours.
- perform one (1) down-route trip as Service 21 from Geylang Road (Bus Stop 80089 - Bef Lor 18 Geylang) towards Saint Michael's Terminal during Sundays & Public Holidays AM Off-Peak & PM Peak, departing at 1815 hours.
- 1 AMDEP Service 21 DD will
- perform one (1) supplementary trip as Short Trip 21A during Sundays & Public Holidays PM Peak, departing Geylang Road (Bus Stop 80089 - Bef Lor 18 Geylang) at 1800 hours.
- perform one (1) down-route trip as Service 21 from Geylang Road (Bus Stop 80089 - Bef Lor 18 Geylang) towards Saint Michael's Terminal during Sundays & Public Holidays PM Peak, departing at 1831 hours.
- SBS3009H, SBS7530S, SG5571Z (Service 20), SBS3513T, SG5479H, SG5480C (Service 81), SBS7577H, SG6084G (Service 22), & several other Service 20 DDs will operate on Service 21 on Weekends & Public Holidays.
- SBS8287M, SBS8289H, SBS8816L, SBS8823R & SBS8830U (Service 21) will operate on Service 59 during Weekends and Public Holidays.
- SBS8322U, SBS8342L, SBS8532E & SBS8667A (Service 21) will operate on Service 20 on Weekends & Public Holidays.
- Majority DD fleet on Weekends & Public Holidays.
- This service uses coloured information plates.
21 (towards Pasir Ris Interchange)
21 (towards Saint Michael's Terminal)
- Weekdays: Ang Mo Kio 12A-12P / 4E / 3S / 2T; Bedok North 7A-6P / 1S (29 buses)
- Saturdays: Ang Mo Kio 12A-13P / 2E / 1T; Bedok North 8A-8P (24 buses)
- Sundays & Public Holidays: Ang Mo Kio 8A-11P / 3S / 4T; Bedok North 6A-7P (25 buses)
- Special Weekdays: Ang Mo Kio 12A-12P / 3E / 4S; Bedok North 6A-6P / 1E / 1T (28 buses)
- Special Saturdays: Ang Mo Kio 11A-13P / 2T; Bedok North 7A-8P (24 buses)
- Special Sundays: Ang Mo Kio 9A-11P / 3S / 3T; Bedok North 6A-7P (25 buses)
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