A computer malfunction caused the closure of both lanes of the Dublin Port Tunnel yesterday afternoon.
It was the first time the tunnel was shut at peak time since it opened in December.
The closure was caused by a fault to the operational sensory system which monitors the height of vehicles entering the tunnel, CO2 levels and alerts the control room to any fire hazards.
The tunnel was shut from just after 1pm for four hours. Gardaí immediately opened the cordon which bans HGVs from Dublin city centre.
While traffic was heavy on the northbound approaches into the city, there was no repeat of the chaos which ensued when a lane of the tunnel was shut just as lorries were coming off ferries in March.
It caused two-hour delays during morning rush hour.
National Roads Authority spokesman Sean O'Neill said the tunnel was closed as a precaution yesterday.
Engineers worked on the system off-line before restoring it at about 5pm.
"It does happen in tunnels around the world because of the new, high-tech sensory systems and the safety procedures that are so high end.
"This is the first time we have had a significant technical issue. Operationally, as the plan was set out for dealing with problems like this, it worked perfectly."
The AA's public affairs manager, Conor Faughnan, said the timing of the shutdown was fortunate as it would have caused chaos at rush hour.
