In the light of a recent case where a bus driver was convicted of drinking while ferrying passengers, bus operators reiterated their protocols on verifying that bus captains are fit to drive before allowing them to start their duties.

These include having staff watch out for signs that a bus captain may be unfit to drive, or having random breathalyser tests.

At Tower Transit, supervisors at the despatch counters where bus captains receive their duties each day look out for signs that the drivers are unfit to drive.

"For instance, as duty packs are handed out... interchange and depot supervisors ask them to repeat the details of their duties as a simple test of alertness or sobriety," said group communications director Glenn Lim.

Should there be signs of alcohol consumption, supervisors will conduct a breathalyser test. There are also random tests. He added that staff have since been alerted to the case, which happened in June last year, as a "timely reminder of our zero-tolerance policy on alcohol consumption".

In that incident, an SBS Transit bus driver, Nagibullah Raja Saleem, 27, had been so drunk that he nodded off at the wheel, missed bus stops and almost crashed his bus. He also ignored the shouts of alarmed passengers.

Although Nagibullah, who worked a split shift, declared that he was fit to drive and was checked when he started his shift in the morning, he started drinking alcohol before his afternoon shift and continued drinking while driving.

He pleaded guilty on Wednesday to one count each of driving in a rash manner, drink driving and using a mobile device while the bus was moving. He was dismissed following the incident.

Ms Tammy Tan, senior vice-president of corporate communications at SBS Transit, said: "Following this incident, we have further tightened procedures to require a second declaration form to be completed if there is a break of several hours in between trips."

Bus operators Go-Ahead Singapore and SBS Transit also told ST that they both conduct random breathalyser tests on captains, adding that whenever they start work, their captains have to declare that they are fit to drive, and are not feeling unwell or under the influence of drugs or alcohol.