This means that for my commute, the
SG reported average speed is usually about 24 mph. In reality, I'm usually driving in the 30-40 mph range. Stops along the way drop the average.
If you're planning a trip, though, it's actually a more useful metric. Any stops along the way have to be accounted for. With a 60 mph average driving speed, my highway trips are often "rated" at 50 or lower mph. The 50 mph average will tell me the actual trip time, and not some academic-paper theoretical time.