My coldest ever was my 7 mile commute from school to home when it was 3F (-16C). I fondly remember it because when I went down on the ice at one point I was going fast enough to slide partway up the exit ramp from the bike path to the street. Good times. That was in 1996. Nowadays I usually drive if it's below 20F (-7C) but a few years ago I did do my current 21 miles to work when it was around 15F.