I like the tool Eden posted, because it was the combination of temperature and humidity, along with left over moisture/puddles that caused the crash that gave our friend a broken hip. It was the end of March and was predicted to go up to 60, so we all had discussed commuting the night before. Unfortunately, our friend was not quite as experienced in commuting as we were. When my DH got up, he saw the temperature and humidity combo and knew there would be black ice, so he nixed the commute, but our friend did not. Thankfully, he's a bit younger than us, so despite a permanent limp and an ugly recovery, his riding is as fast as ever now.
Last night DH commented he saw someone commuting at 5:30 AM, when he was driving to the gym. While the guy was well lit up and dressed for the 20 degree temps, there was ice everywhere! Not just black ice, but patches of visible ice, as well as snow. It was enough to make me be a little cautious in the car, and when I drove home from boot camp, the town had salted several roads. DH said this guy was on a road bike with skinny tires, which I just think is plain stupid in those conditions.