Depending on where you live the traffic could be louder in the summer. For instance since the weather is warmer now, I tend to drive with my windows down and my music turned up. Also I shout obscenities at people smoking cigarettes.
My thoughts on this are that people get busy more when the weather is colder. I have done a non-scientific study among a large group of friends. Most of us were born during the summer. This means are parents were doing it in winter. Perhaps this has some baring.