Traffic collisions leading to significant congestion on Dublin's M50 added to the stress and anxiety of hundreds of Leaving Cert and Junior Cert students hoping to arrive at exam centres on time yesterday.

The State Examinations Commission confirmed it had been contacted by exam superintendents, travelling to exam centres yesterday morning, expressing concern that they too might be late for the exam start time of 9.30am.

In normal circumstances a student arriving 30 minutes after the designated start time would not be admitted to the exam hall but the commission said it would have made provision for students arriving late due to the M50 congestion.

However, it appeared last night that no exam student was significantly delayed by the high number of crashes and incidents which caused chaos on the M50 yesterday morning.

Nicola Hudson of AA Roadwatch said traffic congestion worsened during the rush hours between 7am and 9am.

"At one point the queues stretched over 8km from Ballymun through to Ballinteer . . . the last thing you want to worry about on the day of your exams is a traffic jam."

Yesterday's traffic congestion began before 8am when a four-car collision occurred in the southbound lane at Edmondstown resulting in emergency services attending the scene for over an hour.

In the following hour a car broke down southbound at the Red Cow slip road and another collision occurred southbound between the West-Link toll plaza and the Lucan exit.

Northbound traffic was also affected when two cars and a truck encountered difficulties between Firhouse and Tallaght, and contributed to commuter delays.

At one point queues stretched from the exit for Sandyford Industrial Estate to the West-Link toll plaza.

All in-bound routes to Dublin city were affected, with areas such as the Lucan Road, Navan Road and Naas Road suffering continued congestion hours after the early-morning rush.