General Secretary of the National Bus and Rail Union Dermot O'Leary has said drivers are not qualified to spot whether a wheel nut is tight enough.
"As you can imagine on a dark winter's morning...it's very much a visual check. Our drivers are not qualified to check if a wheel bnut it tight or not."
Dublin Bus has said passenger safety is their number one priority after a wheel fell off a bus in Dublin city centre during the week.
A rear wheel came off the 15b bus at Butt Bridge, close to Liberty Hall on Tuesday afternoon. Nobody was injured during the incident. Passengers were transferred to another bus before continuing on their journey.
The company said it was a completely isolated incident and an investigation was underway.
Dublin Bus said its drivers carry out visual checks on buses every morning.
In addition, there are trained checks every four week and checks using torquing equipment every 25,000 kilometres.
Dublin Bus has a fleet of 950 buses. Maintenance controllers change up to 10,000 wheels each year. The company has bought 320 new buses into the fleet which have replaced older vehicles. The average age of buses in the fleet is seven years.