Gardaí are investigating the cause behind a number of multiple-vehicle collisions which left at least 30 people injured in Co Kildare this morning.
Gardaí said a fuel tanker had overturned thick fog in a tunnel near Kilcullen, but Superintendent Ken Brennan, the regional traffic chief based in Mullingar, said low visibility and speed was also a factor in the collisions.
"At the time of the collisions this morning conditions were extremely hazardous, down to low visibility," he said. "Our opinion would be that they \[drivers] had ample visibility if they would adjust their driving. Unfortunately most motorists didn't adjust their driving to match the road and weather conditions and that contributed to the mayhem."
Witnesses spoke of red brake lights suddenly appearing out of the thick fog, drivers slamming on brakes forcing others to swerve into the central median, hard shoulders and roadside grass verges to avoid collisions.
Motorists in the middle lanes though were worst affected, having nowhere to go. Many people, were motoring along in rush hour traffic on their way to work with no lights on, in poor visibility.
It is thought the first crash happened in dense fog at around 9am.
A minimum of 30 collisions were reported over a two-mile stretch of the M7 and a mile and a half of the M9.
Some 38 ambulances from Dublin, Navan and Tullamore attended the scene at the M7/M9 motorway interchange.
Out of 30 admissions to Naas General Hospital only five remain and one is in a "critical" condition. A sixth patient was transferred to Tallaght Hospital and is believed to be stable.
HSE spokeswoman Dympna Bracken said a major emergency plan has been put into operation at Naas General Hospital to deal with some of the injured.
Both the M7 and the M9 have repoened, according to the AA after traffic tailbacks of 20 kilometres subssided. AA Roadwatch had earlier advised on a number of diversions but all restrictions have now been lifted.