Why speed limit 120 kph?

Answer 1

If cars drive too fast, it can be very dangerous. So it is important to have speed limits. They will differ for different standards of road. A good road that is wide and straight and has lanes separated clearly, with possibly a barrier between traffic going in the opposite direction, are safer. So they can have higher speed limits, than say a narrow road with lots of hills and turns in it. So 120kph is more appropriate for better and safer roads. Even at that speed there are dangers, so letting cars go too fast is more dangerous. A limit has to be at some level, and 120kph is chosen for many roads.