Being hit by a car is a definite trope in Kdrama, but there is a reason for that. Until recently South Korea had one of the highest pedestrian death rates in the world. It's the easiest way to kill off a character very suddenly and it matches the statistics. In American TV shows sudden death is usually by gun violence.
The safety pin they're talking about is like the pin on a grenade that keeps it from exploding. Pull the pin and the grenade will explode in 3-5 seconds.