I notice that below 40 degrees (Farenheit) that my average speed drops about 1 mph for every 10 degree temperature drop, even though it feels like I'm working just as hard. Compound this with the 4 - 5 mph loss incurred by switching to my winter bike with studded tires, and my 7.5 mile commute time doubles from about 1/2 hour (summer) to 1 hour (winter). And this is assuming plowed roads. If I'm fighting a fresh snowfall things get even slower.