My lowest commuting temperature is the one in which my hands and feet become uncomfortable for a 40 minute ride. This temperature changes as the fall progresses. In November it's about 40 F. By the time I hang it up for the winter, I've adapted and it's upper 20s. The wind plays into this, also. I might not ride if it's 40, dark and windy, but if it's 30, sunny and calm, I might.