Transport giant ComfortDelGro Corp has appointed three retired permanent secretaries to its board and the boards of its subsidiaries.

Mr Chiang Chie Foo, who was permanent secretary at the Prime Minister's Office and the Ministry of Defence before his retirement from the civil service in 2013, joins the ComfortDelGro board on May 1.

He is now chairman of national water agency PUB and the Central Provident Fund Board.

Joining the board of subsidiary SBS Transit yesterday was security expert Benny Lim. Mr Lim was permanent secretary at the Prime Minister's Office and the Ministry of Home Affairs and a deputy commissioner of police before retiring from the civil service in 2016.

At vehicle inspection firm Vicom, another subsidiary, Dr Tan Kim Siew, who was permanent secretary for defence development at the Ministry of Defence and an engineer, will join its board on May 1.

Dr Tan is senior consultant at the Ministry of Finance and chairman of the National University of Singapore's (NUS) governing board for the Mechanobiology Institute.

ComfortDelGro also announced the appointments of three other directors: Mr Daniel Soh, a partner of Ernst & Young from 1990 till his retirement in 2012; Dr Ooi Beng Chin, chair professor at the National University of Singapore's Department of Computer Science; and Professor Simon Yu, who is programme director in sustainable infrastructure engineering at the Singapore Institute of Technology.

Directors who have retired are Mr Oo Soon Hee of ComfortDelGro, Mr Cheong Yip Seng and Mr Chin Harn Tong of SBS Transit, and Mr Ong Teong Wan and Mr Sim Cheok Lim of Vicom.

Group chairman Lim Jit Poh said: "Altogether, all these five directors have put in 171 years of service... This is indeed commendable and the group is greatly grateful." The new directors, he said, bring with them decades of experience and a keen understanding of various disciplines.

At its annual general meeting yesterday, ComfortDelGro shareholders asked how the group was responding to competition from private-hire players, about its acquisition of Uber's Lion City Rentals, and its investment yields in the years ahead.

Mr Lim said ComfortDelGro has proven naysayers wrong by posting total shareholder returns of almost 10 per cent since 2003. He said it would not be wise to give specific answers as the firm was in the midst of building up a private-hire business.