Here's why:

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* Speed is the rate at which an object moves, and it's always positive.

* Direction is indicated by a positive or negative sign.

Therefore, if a car is moving backwards, its direction is opposite to the conventionally chosen positive direction, making its velocity negative.

For example, if a car is traveling at 10 m/s backwards, its velocity would be -10 m/s.