You have to hand it to the Koreans. Their road infrastructure is impressive. The entire place is very mountainous, with a great costal area made up of several small islands.
This means lots of tunnels through mountains and bridges over waterways. The scale of some of them is immense! I’d prefer it if you were allowed to ride a bike over them though! Most of them are ok, but the expressway bridges are no bikes.
I know this because I blundered onto one earlier. (It’s to late by the time you realise). The staff at the toll booth looked completely stunned, like a Marian had pulled up in a space ship! They were pretty good though, opening barriers for me to turn around.