Commuters are being urged to allow extra time for their journeys today.
Schools are re-opening after the Christmas break, with many workers on extended holidays also making a return.
Meanwhile, the cold snap in many places overnight is also likely to have an impact on traffic.
"Generally speaking you are going to see traffic be that bit busier on most of the key routes across the country - in part because of the return of school traffic," said Barry Aldworth from the AA.
Current temps (°C at 6:30am)
— Met Éireann (@MetEireann) January 8, 2018
Mt Dillon (Rosc) -6.5
Gurteen (Tipp) -6.0
Athenry (Gal) -5.9
Claremorris (Mayo) -5.1
Ballyhaise (Cav) -4.8
Moore Park (Cork) -4.4
Shannon Airport -4.1
Knock Airport -2.9
Mullingar (Wes) -2.4
Dublin Airport -1.7
Cork Airport 0.8
Valentia (Kerry) 4.0
"So last week and particularly the week before the Christmas period, it would have been quiter on the roads.
"So you would have had less cars, people would have been taking time off themselves or extended time off with children who were also on their holidays."
The cold weather will continue on into this afternoon with most places seeing highs of just 2-5°C with the south and southwest coast reaching 6-7°C.
Winds will increase this evening and into the night bringing fresh and strong Atlantic gusts.
Lowest temperatures tonight will be around 1°C but will gradually increase through the night to reach 6°C in some parts.