Female labor force participation down
In the Washington Post article: Whither the Women? it is noted that since the turn of millenium the percentage of women in the labor force has been dropping. This has many important implications for travel demand.
In particular, travel demand is often divided into work and non-work trips, and work trips tend to be peaked in the morning and afternoon, while non-work are more spread throughout the day (with some significant chaining on the trip to and from work by workers). If fewer people (men or women) work, then peaking will diminish, and there will be a heavy but more consistent level of traffic throughout the day. Unfortunately (for traffic anyway) nonworkers still travel a lot, though not quite as much as workers. See:
for some analysis of data on the issue.