Yes, if a car is moving at a steady speed, its horizontal velocity is constant. Velocity is a vector quantity that describes both the speed and direction of an object's motion. When an object moves at a steady speed, it means that its magnitude of velocity (i.e., speed) remains constant, and consequently, its direction remains unchanged.
The horizontal velocity specifically refers to the component of the velocity that is parallel to the horizon or in the x-direction. If a car is moving on a flat surface without any changes in its speed or direction, the horizontal velocity remains constant. However, it's worth noting that external factors such as friction, air resistance, or changes in terrain could potentially affect the velocity and lead to variations. But under ideal conditions, assuming no external influences, the horizontal velocity of a car moving at a steady speed remains constant.