On Korean long-distance buses, plan around the highway rest stop rather than an onboard restroom. VISITKOREA's current express/intercity bus guide states that these buses do not have onboard restrooms and that mid- to long-distance routes include scheduled breaks at highway service areas, called hyugeso.
The same official guide describes common expressway rest-area facilities such as public restrooms, nursing rooms, tourist information centers, restaurants, convenience stores, gas stations and EV charging stations. Some locations also have themed food markets or leisure facilities. That makes a rest stop useful for more than a toilet break, but it does not mean every service at every location is open at every hour.
If you are a bus passenger, listen for the announced break length and return time, remember exactly where the bus is parked, and take valuables with you. Do not assume the driver will wait for a late passenger. If you need a restroom urgently, it is better to use the scheduled break promptly than to expect another unscheduled stop.
If you are driving a rental car, use rest areas proactively for fuel or charging rather than waiting until the battery or tank is nearly empty. If a specific service matters—EV charging, nursing facilities, a staffed information desk or a late meal—check the exact service area before the trip because operating availability can differ.
A simple rule: for buses, know the return time; for cars, know the next usable stop. The food can be part of the fun, but the operational value is reliable access to a safe break on a long road journey.