The speed of light being approximately 299,792 kilometers per second is a real physical fact based on scientific measurements and experiments. It is a fundamental constant of nature and plays a crucial role in the theory of special relativity, proposed by Albert Einstein.
The example you provided about two cars passing each other on a highway is not directly related to the speed of light, but it does illustrate a concept called the "apparent speed" or "relative speed" in classical mechanics. When two objects are moving close to each other at speeds that are comparable, the apparent speed difference between them can indeed appear to be larger than their actual speed difference.
This effect occurs due to the reference frame from which the observation is made. In the scenario of two cars passing each other, if you are in one of the cars, the relative motion between the two cars will be more pronounced if they are closer together. Your perspective from inside one of the cars affects how you perceive their motion relative to each other.
However, when dealing with the speed of light, it behaves differently because it is an absolute constant in all inertial reference frames according to the principles of special relativity. No matter how fast an observer is moving relative to a light source, the speed of light will always appear to be the same for them. This unique characteristic of the speed of light is one of the fundamental principles of modern physics and has been confirmed by numerous experiments and observations.