From Jim Foti in the Strib: Lost jobs add up to speedier commute
One might add high gas prices (and the stickiness that results when people changed behavior in response), market saturation (how much more could we travel per capita anyway), and decreased non-work travel (associated with decreased spending) as additional factor besides unemployment which are resulting in reduced VMT and congestion.
Or, people tend to more often use public transportation in order to save money. As you also indicated, further research can compare the traffic volumes in rush/nonrush hours before and after the recession. My hypothesis is that the drop of traffic volume in nonrush hours (mostly representing non-work travel) may be greater.