...for example if you say the average 12 km/h , it doesn't mean that you constantly drive with 12 km/h.
Many measurements in real life imply averaging - at least because of the limited precision with which we can measure distance and time. The speed that a car speedometer shows is an average over a small distance and small time. Although the fluctuations about average are usually rather small, it is fair to say that the speed of a car is not always constant, even when the arrow/number on the speedometer doesn't move.
In older cars one could actually see that it takes time for the speedometer to settle to a particular number after a period of acceleration or even see its arrow shaking when driving. In most modern cars the averaging time is much shorter - at least shorter than a human reaction time (about a quarter of a second), so that the fluctuations of speed are not perceptible - all we observe is a speed average over our reaction time. (In addition, the fluctuations are small, if using cruise control.)
Finally, in a different context the speed shown by a speedometer may be considered as instantaneous: e.g., if we are talking about speed variation during a long trip (e.g., from New York to Denver), and we are not trying to be more precise than, say $0.1$km/h. In this case average speed can be still a meaningful parameter - e.g., when calculating how much time it will take you to get from Denver to Los Angeles or how much gas your car spends per kilometer/mile.
Remark: a textbook matter, but worth reminding - the average speed and average velocity are not the same thing.