I second the suggestion to take public transit in Seoul, even if you do not do so at home.
Traffic in Seoul is just plain awful. Despite the subways, which are extensive, fast, and convenient to everywhere, Seoul differs from Tokyo in that the subways are supplemented entirely by buses instead of partly by surface and elevated trains.
I took one of those hop-on, hop-off tour buses, and it kept getting stuck in traffic. This experience convinced me to buy a T-Money card and rely on the subway and my feet as much as possible.
However, when I took the bus from Seoul Station to ICN, as my guesthouse owner recommended, it ran on a dedicated busway until we were out of the city, so traffic was no problem.