I do think we have a limit based on our body makeup, but we can push ourselves to improve. I think that certain training techniques work better for each of us, so it's finding what will increase your speed. I also think it's a function of your bike. I have a Trek 1200 and Lemond Zurich. When I was on the Trek I was averaging around 17mph. On the Lemond I average close to 20. I don't think I changed that much in my technique to justify the increase in speed. I guess also, how many hours a day/times per week do your friends train versus you? They may have more time in the saddle which could explain their increased speed.