Google Maps automatic speed detection

Today I went for a five-mile bike ride with my family along the Charles River to Harvard Square. Midway, my father checked google maps to see how much more time it would take to reach. It said that it would take 16 more minutes to reach by biking. Since my biking speed is in between the average walking speed and the average biking speed, it took me about double the time to reach Harvard Square than the GPS said. I came up with an idea on the way that the GPS should have a way that it detects the speed automatically even while biking or walking. Then it should calculate the time based on the speed it detected. This will be helpful for everyone because we all walk and bike at slightly different speeds.