During peak season and even weekends, traffic can get really bad near major attractions like Nami Island or Jinhae or anywhere in their prime time such as Garden of Morning Calm during their light show or Ski Resorts on Holidays/Weekends.
Usually, it's not the highway that gets jammed, it's the entrance to the venue. The reason being that many of these attractions have very small parking lots and narrow roads leading up to it. It will easily take up to an hour just to get to the parking lot from the main road.
The later you leave Seoul or Busan, the worse it gets. Since everyone will be on the road with you.
"Early bird gets the worm!"
For example, it normally takes 1 1/2 hours to reach Nami Island from Seoul on a weekday during the off-season (Which is also the estimated travel time that Naver Maps will quote).
However, during the peak season and/or weekends, it may take as much as the following:
Leave Seoul at 7:00 AM - 2 ~ 2 1/2 hours
Leave Seoul at 8:00 AM - 3 ~ 3 1/2 hours
Leave Seoul at 9:00 AM - 4 ~ 4 1/2 hours
Leave Seoul at 10:00 AM - 5 ~ 6 hours
*Of course, these are just estimates and may not reflect the actual travel times but this is based off past experiences of clients and staff alike*
Our best advice is to avoid the traffic condense times. If that's not possible, hit the road before everyone else!