I've seen this in certain parts of the region. My guess would be that the evening rush hour is more compressed than the morning rush hour. Specifically, in the morning people tend to come in over the course of about four hours (5:30 am to 9:30 am). In the evenings, while some people start leaving around 3:30 pm, the real crush comes between 5pm and 6pm.
When I leave my office between 4pm and 5pm, the rush hour is usually pretty tolerable. Same goes for leaving the office after 6pm. That 5pm to 6pm hour is a bitch no matter where you drive from.
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http://bible.cc/1_corinthians/13-11.htm