The other thing to consider is that some people use bicycle computers that only count rolling time. If the bike is standing still, it doesn't enter into the rider's total time of the ride. This is, in my opinion, cheating at least a bit, because if a rider just finished climbing a hill, then gets a chance to rest at a red light, they will be able to go faster than if they rode straight through. In my case, I look at Google maps after a ride (if I don't know the distance) and then divide by the time I was out riding. Based on that I average right around 13 mph.