Common sense has been evident in the posts that responded to Dangerous Dave. In initiating the question, he was seeking a precise measure, "... a temperature below which I absolutely ...". There is no precise measure which is why there is a variability in responses including that of Dangerous Dave's brother/doctor. Along with air temperature, consideration must also be given to wind, hence wind chill. The posters who suggested knee and leg warmers are obviously accustomed to the concept of layering of clothes. When temperatures vary greatly during a day, the prepared cyclist/commuter will have a layer of clothes to respond to the situation. Pictures of cyclists together during cool/cold temperatures invariably reveal a variety of clothes for reasons that individuals react subjectively and differently to the weather.