Seoul is a concrete jungle where the vast majority of people live in tiny concrete apartments. (Literally concrete, I destroyed a lot of masonry drill bits trying to hang pictures on the concrete walls of our apartment.) Sure the subway network in Seoul is extensive but it's still not exactly a fast way to travel.
Speaking from experience the fastest way to get around Seoul is on a scooter. That's because scooters routinely jump red lights, go the wrong way down one-way streets and take shortcuts on sidewalks with impunity. Lots of fun when you're young with no kids, as I was at the time, but it doesn't scale and would never be allowed in LA.
The easiest way to cut down on vehicle traffic is to stop forcing people commuting to workplaces where they sit down at an internet connected computer that they could just as easily do at home. As the current pandemic has forced so many of us to do, which is one big upside.