Today is Lunar New Year's Day, an occasion celebrated in many parts of Asia.
In Korea, it's called "Seollal," and for most people it's an extended break that'll last until Wednesday.
Millions of people are leaving the big cities and heading to their hometowns to spend time with family, perform ancestral rites and eat traditional food.
With so many people on the roads, though, traffic tends to build up.
It's expected to peak around 4 to 5 PM.
The Korea Expressway Corporation estimates that from Seoul it'll take about 5 hours to get to the southeastern city of Busan... and about 4 hours to Mokpo, in the southwest.
Officials say the congestion will last until late tonight... as people head home after visiting their ancestors' graves.