Why is the initial speed of a car at a red light 0 m/s?

The initial speed of a car at a red light would be 0 m/s, because it has fully been put to a stop. It's not moving.

Step-by-step explanation:

If a car were to fully stop there would be no speed.

For example, if you were to go walking in the city, and you were about to walk onto the crosswalk, but the signal turn into a reg light. Then you would have stopped. Then you have gone into an idle position. When in this idle position, it is like you aren't moving, so there is no acceleration in speed or decrease. It is also much like a stop sign, you would make a full stop when you see it.