Good weather congestion.

In northern cities where winters can be harsh, most people take advantage of any good weather given by nature specially in winter months. I noticed here in Chicago that if it is sunny and in the high 30s to low 40s during winter, everybody take advantage of the relatively good weather and flock outside to go shopping, walking, etc. In the Spring and Fall if the weather is in the high 60s or low 70s, same phenomenon happens. More people are in the street or in their car out to go somewhere outdoorsy. This adds up to rush hour traffic which is kind of annoying when you are just trying to get home. But then again, one should be thankful of the nice weather.