HelenP My two cents. On my lightest road bike my average commute speed is 13.5 mph. On my old MTB I average 11.5 mph with my other bikes falling in between. While the average speed varies a little, the biggest factor in ride time is traffic lights and traffic crossings, which erases any speed advantages for a particular bike.
On the days where I decided to really push it I've raised the average speed to as much as 15 mph which only netted me a 5 minute savings over 9 miles of street riding.
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timdow after hitting 45 I would gladly trade speed for comfort, and now at 60 comfort and safety are even more important. The idea of daily exercise is my goal now.
As far as getting passed, I live in Colorado Springs, home of an Olympic Training Center and Velodrome. I get passed constantly. What makes up for it is being able to live life with the same health, (almost) stength, stamina and flexibility I enjoyed at 35.